C-reactive protein (CRP) is one of the most studied and most used laboratory tests for neonatal sepsis. As part of the acute-phase reaction to infection, it plays a central role in the humoral response to bacterial invasion. The delayed synthesis during the inflammatory response accounts for its low sensitivity during the early phases of the disease. Diagnostic accuracy clearly improves by the performance of serial determinations and by the combination with earlier markers such as interleukins or procalcitonin. CRP is as well particularly useful for monitoring the response to treatment and guiding antibiotic therapy, though nothing replaces the clinical impression and the gold standard (i.e. culture results). In spite of the large amount of research done on CRP in neonates, some topics are still not fully understood, such as the influence of noninfectious factors on CRP levels in healthy as well as in symptomatic neonates and the role of gestational age and birthweight on CRP kinetics. In this review, we aim to give an update on the current evidence on the use of CRP in neonates.

Despite the advances in neonatal care, early-onset neonatal sepsis remains a serious and potentially life-threatening disease with a mortality rate ranging from 1.5% in term to almost 40% in very-low-birthweight infants [1,2]. The signs and symptoms of neonatal sepsis may be subtle and nonspecific being clinically indistinguishable from various noninfectious conditions such as respiratory distress syndrome or maladaptation. The current practice of starting empirical antibiotic therapy in all neonates showing infection-like symptoms results in their exposure to adverse drug effects, nosocomial complications, and in the emergence of resistant strains [3].

Laboratory sepsis markers complement the evaluation of clinical signs and risk factors in diagnosis of neonatal sepsis. No currently available test is able to provide perfect diagnostic accuracy, and false-negative as well as false-positive results may occur; the usefulness of a laboratory test therefore primarily depends on the clinical condition of the child. For example, in a critically ill newborn a negative test result will not give much additional information on its infectious status, and in an apparently well infant a positive result will not dramatically increase the probability that the child is infected. Diagnostic tests will be most useful in infants with clinically unclear infectious status [4]. There is great interest in rapid diagnostic tests that are able to distinguish infected from uninfected newborns, especially in the early phase of the disease [5]. In fact, a delayed start of the antibiotic treatment may be no more able to stop the fulminant clinical course with development of septic shock and death within hours after the first clinical symptoms [6]. In the era of multi-resistant microorganisms, it is also important to avoid the unnecessary use of antibiotics in sepsis-negative infants.

There is an abundance of studies evaluating laboratory markers in the diagnosis of neonatal sepsis. Despite the promising results for some diagnostic markers, current evidence suggests that none of them can consistently diagnose 100% of infected cases. C-reactive protein (CRP) is the most extensively studied acute-phase reactant so far, and despite the ongoing rise (and fall) of new infection markers, its wide availability and its simple, fast, and cost-effective determination make it one of the preferred indices in many neonatal intensive care units (NICUs) [7].

CRP was first described in 1930 by Tillet and Francis at Rockefeller University [8]. They observed a precipitation reaction between serum from patients suffering acute pneumococcal pneumonia and the extracted polysaccharide fraction C from the pneumococcal cell wall. This reaction could not be observed when using serum of either healthy controls or the same pneumonia patients after they had recovered. In view of the fact that the polysaccharide fraction was a protein, the C-reactive component in the serum was named C-reactive protein [8]. By the 1950s, CRP had been detected in more than 70 disorders including acute bacterial, viral, and other infections, as well as noninfectious diseases such as acute myocardial infarction, rheumatic disorders, and malignancies [9]. All of these disorders of disparate etiology had in common the theme of inflammation and/or tissue injury [10].

The principal ligand to CRP is phosphocholine, which is found in lipopolysaccharide, bacterial cell walls, as well as in most biological membranes [11]. After binding, CRP is recognized by the complement system; CRP activates it, and promotes phagocytosis of the ligand by neutrophil granulocytes, macrophages, and other cells. CRP further activates monocytes and macrophages, and stimulates the production of proinflammatory cytokines [11,12].

CRP is part of the acute-phase response, a physiological and metabolic reaction to an acute tissue injury of different etiologies (trauma, surgery, infection, acute inflammation, etc.) which aims to neutralize the inflammatory agent and to promote the healing of the injured tissue [10].

After a trauma or the invasion of microorganisms, an acute inflammatory reaction is initiated by activation of local resident cells which promote the recruitment and activation of further inflammatory cells, including fibroblasts, leukocytes, and endothelial cells. Once activated, they release proinflammatory cytokines including IL-1, TNF-α, and IL-6. These cytokines induce the production of proteins of the acute-phase response in the liver. These include but are not limited to components of the complement system, coagulation factors, protease inhibitors, metal-binding proteins, and CRP [10,12].

The production of CRP in the hepatocytes is mainly induced by IL-6, but can be further increased by synergy with IL-1 [13]. In 1981, Shine et al. [14] evaluated serum concentration of CRP determined by radioimmunoassay in 468 sera from normal adult volunteer blood donors, and reported on a median concentration of 0.8 mg/l with a 90th percentile of less than 3.0 mg/l. More recently, Rifai and Ridker [15] used three different high-sensitivity techniques to determine CRP distributions in their cohort consisting of 22,000 healthy adults from the United States. The median CRP values for men and women were 1.5 and 1.52 mg/l; the 90th percentiles were 6.05 and 6.61 mg/l, respectively. Similarly, Imhof et al. [16] examined CRP values from 13,000 apparently healthy men and women from different populations in Europe. The reported median concentration in the single cohorts ranged from 0.6 to 1.7 mg/l, the 90th percentiles from 3.2 to 8.0 mg/l.

During the acute-phase-response, CRP’s hepatic synthesis rate increases within hours and can reach 1,000-fold levels [9,11]. Levels remain high as long as the inflammation or tissue damage persists and then decrease rapidly. The half-life time has been reported by Vigushin et al. [17] to be 19 h in any of the diseases studied, being the fractional catabolic rate independent of the plasma CRP concentration. From this information, the synthesis rate of CRP therefore appears as the only significant determinant of its plasma level, supporting the clinical use of CRP measurements to monitor disease activity in all disorders characterized by a major acute-phase response.

Any elevation of serum CRP in the neonate always represents endogenous synthesis, since it passes the placenta in exceedingly low quantities [18]. De novo hepatic synthesis starts very rapidly after a single stimulus with serum concentrations rising above 5 mg/l by about 6 h and peaking at around 48 h [19].

For the diagnosis of early-onset sepsis in clinical practice, the sensitivity is more important compared to the specificity, as the consequences of unnecessarily treating an uninfected infant bear fewer complications than not treating an infected child.

In diagnosis of early-onset sepsis, previous studies reported on widely differing sensitivities and specificities of CRP ranging from 29 to 100% and from 6 to 100%, respectively [10,20,21]. These extreme variations are a result of different reference values, a posteriori-selected cutoff points, test methodologies, patient characteristics and inclusion criteria, as well as different definitions of sepsis, numbers of samples taken, and sampling times.

The sensitivity of CRP is known to be the lowest during the early stages of infection [22,23,24]. For a single CRP determination at the time of initial evaluation as well as for determinations from cord blood, the CRP diagnostic accuracy varies widely within an unacceptable range of sensitivity [22,23,25,26,27,28,29,30,31,32]. This may be related to the arbitrary choice of optimal cutoff points [27,28,30,33] as well as the insensitive analytic methods with various limits of quantification used in the past [34] to detect the CRP pattern in the earliest course of infection, in particular in the very early neonatal period.

A raised CRP is not necessarily diagnostic for sepsis, as elevations may also occur due to the physiologic rise after birth or noninfection-associated conditions (see below). Therefore, concerns were raised about the reliability of CRP during the early stage of the disease being neither able to diagnose nor to rule out an infection with certainty [22].

Benitz et al. [22] found that the sensitivity in the diagnosis of culture-proven early-onset sepsis increased from 35% (95% confidence interval 30–41%) at the initial sepsis workup to 79% (72–86) after 8–24 h, and 89% (81–94) for the higher of two levels obtained after 8–48 h after the initial workup. Concurrently, they reported a decrease in specificity from 90% (88–92) to 78% (76–81) and 74% (71–77) for CRP levels performed as described above. Pourcyrous et al. [23] evaluated serial CRP levels in a large series of 689 investigations for neonatal sepsis (187 of them with positive blood culture results) in 489 neonates, and determined CRP at the initial sepsis evaluation and 12 and 24 h later. The postnatal age at the time of the initial investigation ranged from less than one day (60%) to 191 days (infants were older than one month in 13%). They [23] reported a higher sensitivity for any of the three CRP values (obtained by three serial determinations at 12-hour intervals) compared to the first value (74 vs. 55%). In general, the sensitivity substantially increases with serial determinations 24–48 h after the onset of symptoms [10,20]. Several studies reported on sensitivities and specificities ranging from 74 to 98% and from 71 to 94%, respectively, for either serial CRP determinations or a single determination at least 12 h after the onset of symptoms [22,23,24,25,26,33]. However, by that point most newborns will be asymptomatic and will have confirmed negative culture results [35].

Philip [36] and later others [37,38,39,40] suggested that serial levels may also be useful for identification of infants who do not have a bacterial infection. A repeat CRP 24–48 h after the initiation of antibiotic therapy has been reported to carry a 99% negative predictive value in accurately identifying, in the early neonatal period, infants not infected [7,20,22,41,42,43].

Serial CRP measurements can be helpful in monitoring the response to treatment in infected neonates, to determine the duration of antibiotic therapy, and to recognize possible complications [23,24,44]. In a cohort of 60 neonates with early-onset sepsis, Ehl et al. [45] demonstrated that after initiation of a successful antibiotic therapy, CRP values further increased, peaking and consecutively decreasing after 16 h. A CRP level that returned again to the normal range may indicate that the duration of antibiotic treatment has been sufficient, allowing discontinuation of antibiotics [41], provided the clinical condition of the child improved and culture results were negative.

Thus, CRP has been proposed as a key decision parameter for guiding the duration of antibiotic therapy [20,22,41,42,43]. However, CRP was not the single criterion evaluated in any infant in these studies. In fact, other criteria explicitly included in the decision of whether or not to discontinue antibiotics were clinical status, culture results, and results of other laboratory tests. Thus, the current literature does not sustain CRP as the single decision parameter to discontinue antibiotics.

The magnitude of the CRP response to sepsis was reported to depend also on the underlying pathogen. In 1974, Sabel and Hanson [46] reported that Escherichia coli infection increases CRP levels with impressive consistency. In 1993, Pourcyrous et al. [23] reported the same phenomenon among 187 cases of positive blood culture in 691 investigations for sepsis in infants aged from birth to 191 days of life. They added evidence on a more distinct CRP increase in Gram-negative compared to Gram-positive strains with CRP levels above 10 mg/l in 92 and 64% of 174 single-organism blood cultures, respectively. Cultures with growth of Escherichia coli, group B streptococci, and Staphylococcus aureus were associated with abnormal CRP values in 100, 92, and 89% respectively. The percent incidence of abnormal CRP concentrations varied considerably among the organisms recovered with persistently normal CRP levels in 48 of 174 single-organism blood cultures. In 40 of them (36 with Gram-positive strains, mainly group D streptococci, Streptococcus viridans, and Streptococcus epidermidis), antibiotic therapy was not administered or inadequate. All of them had uneventful clinical courses, and thus these positive blood culture results may be caused by contamination [23]. Other reports on a pathogen-dependent CRP response in neonates include that of Rønnestad et al. [47] who evaluated CRP responses from day 1 to day 4 in 121 monomicrobial septic episodes in neonates. They reported significantly lower median values (day 1–4) in coagulase-negative staphylococci (23 mg/l) compared to S. aureus, group B streptococci, and E. coli (51–58 mg/l).

Interpretation of CRP in diagnosis of early-onset sepsis may be hindered by several noninfectious conditions influencing values during the days after birth (see table 1).

Table 1

Noninfectious conditions influencing CRP values during the first days of life

Noninfectious conditions influencing CRP values during the first days of life
Noninfectious conditions influencing CRP values during the first days of life

In adults, CRP elevation is known to be associated with a large variety of disorders apart from bacterial, viral, and fungal infections including burns, surgery, rheumatic disorders, malignancies, and vasculitis [9].

However, the issue of noninfectious CRP elevations in the neonate is not undisputed; the available data are to some extent contradictory, and, despite the amount of studies on this issue, inconclusive. The earliest descriptions of noninfectious conditions influencing CRP derive from simple observations that elevated values in not infected infants might be connected to coincidental noninfectious conditions, though no statistical confirmation is given.

In 1982, Ainbender et al. [40] described CRP values >20 mg/l in 11 of 100 uninfected infants consecutively admitted to the special care nursery. The authors described that 8 of the 11 infants had, either singly or in combination, shock, meconium aspiration pneumonitis, fetal distress, maternal fever and PROM, and none was found to be infected. Forest et al. [48] reported elevated CRP values between 11 and 70 mg/l in 16/49 uninfected neonates admitted to the NICU with diagnoses of intraventricular hemorrhage, meconium aspiration pneumonia, anoxic encephalopathy, PROM, respiratory distress syndrome, chorioamnionitis, aspiration pneumonia, and transitory tachypnea. There exist further studies that were conducted similarly (see table 1), but rather small sample sizes and the lack of statistical confirmation impair their reliability.

Identifying independent variables that influence the interpretation of CRP in symptomatic neonates or in neonates at risk for infection may represent an important aid in the differential diagnosis of early-onset sepsis. In 2004, Mathai et al. [49] evaluated CRP values in 250 infected and uninfected neonates, and found maternal fever and prolonged labor being significantly associated with values >12 mg/l at 24 h of life. Chiesa et al. [31] aimed to identify independent factors influencing CRP in 134 critically ill infected and uninfected neonates by multiple linear regression analysis, and described nonsignificant association with both of the above described conditions and with other, mainly maternal and perinatal factors, at birth, at 24 and 48 h of life.

Few investigations were performed on the association of CRP with noninfectious conditions in healthy neonates. Chiesa et al. [50] evaluated conditions influencing what constitutes normal CRP values in healthy neonates. In their analysis on 148 healthy term or near-term neonates, they identified low 5-min Apgar score and premature rupture of membranes being significantly associated with CRP response at birth and pregnancy-induced hypertension with CRP response at 24 h of life. In a similarly selected cohort of 421 healthy neonates including 200 premature infants, they confirmed an association with the time of ruptured membranes and added duration of active labor, prenatal steroids, and intrapartum antimicrobial prophylaxis as variables that had a significant effect on CRP concentrations when adjusted for gestational age, gender, and sampling time [51].

Even though there exist numerous studies on noninfectious factors influencing CRP in neonates, the wide-ranging inclusion criteria and thus study populations (infected, uninfected, symptomatic, healthy, at risk, critically ill), differences in methodology, upper limits for CRP, influencing factors chosen to analyze and their definitions hinder the comparison of these studies and impair drawing generally applicable conclusions.

The current literature suggests that CRP may be elevated in some noninfectious conditions, of which some may per se clinically mimic a bacterial infection as well. Thus, the up-to-date available information lacks robust evidence to support a claim that withholding antibiotics may be justified in infants with raised CRP in the above-mentioned conditions.

Even though advances in neonatal intensive care have led to increasing preterm birth rates and survival rates, the influence of prematurity on laboratory test results have been poorly investigated, and have not been assessed systematically. This is also true for CRP, which is one of the most extensively studied infection markers in the neonatal period. Reports on the influence of gestational age and birthweight on kinetics of CRP in infected and uninfected infants are limited.

In a pilot study, Turner et al. [52] demonstrated an association of gestational age with the magnitude of clinically relevant CRP responses during the first 7 days after birth. In case of a clinically relevant CRP rise >10 mg/l, the proportion of a pronounced response >60 mg/l increased with gestational age from 8% in newborns from 24 to 27 weeks to 25% in newborns from 40 to 41 weeks. This pattern of results was also found when infants were grouped according to birthweight.

In a cohort of 348 infants, Kawamura and Nishida [24] reported a lower sensitivity of CRP in the diagnosis of neonatal sepsis in preterm compared to term newborns (61.5 vs. 75%).

Doellner et al. [53] described a significantly lower CRP increase induced by infection in NICU preterm compared to NICU term infants. In their cohort of 42 newborns with either culture-proven or probable sepsis, infants with a gestational age <35 weeks had lower median CRP values and lower CRP peak values compared to infants with a gestational age greater than 35 weeks (median CRP 0 vs. 18 mg/l, CRP peak 15 vs. 52 mg/l).

We have recently reported on a lower CRP response to infection in preterm compared to term newborns with a lower sensitivity (53 vs. 86%), lower median values (9 vs. 18.5 mg/l), and a lower area under the receiver operating characteristics curve (0.799 vs. 0.890) [54].

Some previous studies have addressed the correlation of CRP values with gestational age and/or birthweight in uninfected newborns.

In 2002, Ishibashi et al. [55] demonstrated that birthweight is independently associated with high sensitivity CRP values within 48 h after birth in a cohort of 110 uninfected symptomatic newborns. In 2011, in another cohort including 499 uninfected newborns hospitalized in a NICU, we have reported CRP values determined within the first 3 days of life being significantly lower in preterm compared to term newborns (0.5 vs. 2 mg/l) [54]. Finally, Chiesa et al. [51] have analyzed CRP values in 421 healthy term and preterm newborns from birth to 4 and 5 days of life (for term and preterm neonates, respectively). They found that the healthy preterm babies have a lower and shorter CRP response compared with that in healthy term babies, demonstrating the independent effects of prematurity on CRP values. Mean CRP values increased by 6.0% per week of gestational age at delivery and by 2.4% per 100 g increase in birthweight.

For neonates, assessment of laboratory tests occurs within a complex context of prenatal growth and neonatal development [56]. Though the current literature reveals some minor disagreement on the effect of gestational age on CRP, there is a growing body of evidence suggesting that the so far reported CRP performance may be different among neonates born preterm and term as well as among neonates with low and high birthweight [51,56]. Prematurity of the organ systems and maturational changes in the immune system might result in a more distinct CRP response to delivery in uninfected newborns and to bacterial invasion in infected newborns. The few studies so far addressing this issue suggest that the diagnostic accuracy of CRP in preterm infants may benefit from a reevaluation of the reference intervals and, thus, upper limits of the normal range in this age group [51,53].

Especially in the early neonatal period, many physiological and metabolic processes change and differ from every later moment in life. These changes affect several laboratory parameters as well, and many reference values and serum kinetics substantially differ from later time periods [57].

Reliable reference values are crucial for obtaining an adequate diagnostic accuracy. Upper limits for CRP during the first days of life have mainly been established from uninfected but symptomatic neonates. The few studies assessing upper limits in healthy neonates were mostly based on rather small sample sizes or did not take into account their postnatal age. In 1984, Gutteberg et al. [39] reported on CRP levels of 16 apparently healthy neonates with 5 mg/l being the 97.5th percentile during the first month of life. Forest et al. [48] analyzed CRP values in a cohort of 69 newborns with a normal postnatal course, and reported 68 of them maintained CRP values <10 mg/l during the follow-up period of up to 18 weeks. In 1992, Schouten-Van Meeteren et al. [58] found that 95% of 38 apparently healthy neonates had CRP values ≤10 mg/l taken 12 and 24 h after birth. However, rather small sample sizes, significant error rates of the determination methods used and only partially given follow-up data on the initially apparently healthy neonates represent weaknesses of the above-mentioned studies [4].

Cutoff values reported in the literature range from 1.5 to 20 mg/l with wide-ranging sensitivities and specificities [4,10]. The up-to-date most used upper limit for CRP during the first days of life of 10 mg/l has been established in 1987 by Mathers and Pohlandt [27]. One decade later, Benitz et al. [22] evaluated this cutoff value in 1,002 episodes of suspected early-onset sepsis and confirmed the value being an appropriate threshold above which results can be considered abnormal.

Use of CRP in the first few days after birth is complicated by a nonspecific rise primarily related to the stress of delivery [10,50]. In 1993, Kääpä and Koistinen [18] described significantly higher CRP values in neonates after 24 h of life compared to values taken immediately after birth (p < 0.001). Thereafter, neonatal CRP values remained unchanged for the first 3 days of life. In 2001, Chiesa et al. [50] aimed to assess the ‘normal’ dynamics of CRP after birth. In their cohort of 148 neonates with an unremarkable clinical course from birth to the 4-week follow-up visit, the 95th percentiles of CRP values at birth, 24, and 48 h life were 5.0, 14.0, and 9.7 mg/l. In 2011, Chiesa et al. [51] examined the postnatal reference intervals separately for term and preterm neonates, and described a lower and shorter CRP rise in preterm compared to term infants with peak values of 11 mg/l and 13 mg/l, respectively.

These observations raise concern about the static cutoff value not reflecting the physiologic kinetics of CRP after birth. At the moment, there exist some studies that have aimed to determine age-specific reference values for CRP during the first days of life. [31,55]. However, these values are still to be validated in an independent patient cohort.

An important limitation of CRP is the low sensitivity during the early phases of sepsis. CRP takes 10–12 h to significantly change after the onset of infection [7]. Earlier in the inflammatory cascade, activated macrophages release proinflammatory cytokines and growth factors. Their increase therefore precedes the changes in CRP. Of the many mediators studied, much attention has been focused on IL-6, IL-8, and TNF-α.

IL-6 increases rapidly after the bacterial invasion, and was demonstrated to have a high sensitivity during the early stages of sepsis (80–100%) even when determined from umbilical cord blood (87–100%) [28,59,60,61]. Chirico and Loda [7] report that the IL-6 short half-life results in a rapid normalization of its serum levels, even though the infection persists. However, Panero et al. [62] demonstrated that NICU neonates with clinical and/or microbiologic evidence of infection have persistently elevated IL-6 compared to neonates without clinical and microbiologic evidence of infection. Chiesa et al. [50] described significantly higher IL-6 concentrations in healthy near-term (35–36 weeks of gestation) compared to term neonates (≥37 weeks of gestation) at birth and at 24 and 48 h of life, thus suggesting a gestational age-dependent effect on IL-6 values over the first 48 h of life. IL-8 and TNF-α have very similar characteristics and kinetic properties to IL-6 [59]. While studies report on a reliable diagnostic accuracy of IL-8 with a sensitivity of 69–100%, the usefulness of TNF-α as a diagnostic marker has not been found to be as good as IL-6 or IL-8 [59,60]. IL-8 was described to vary with gestational age, with preterm infants having a wide variety of associated comorbidities and thus higher IL-8 values compared to term infants as determined in cord blood [63] and serum collected on postnatal days 2–5 [64]. As such, the independent effect of gestational age on IL-8 neonatal response has yet to be assessed.

Procalcitonin (PCT) is an acute-phase reactant which has the advantage of increasing rapidly after contact to bacterial endotoxin with levels rising after 4 h and peaking at 6–8 h [59,65]. In a recent meta-analysis, the sensitivity and specificity in the diagnosis of early-onset sepsis were 76% (range 68–82) and 76% (60–87) [66]. In a recent single-center, prospective, randomized intervention study, Stocker et al. [67] have analyzed the effect of PCT-guided decision-making on duration of antibiotic therapy in suspected neonatal early-onset sepsis and reported significant higher proportion of infants treated for less than 72 h in the PCT-guided compared to the conventional group (45 vs. 18%).

Specific leukocyte cell surface antigens are known to be expressed in substantial quantities after inflammatory cells are activated by bacteria or their cellular products [68]. They have the advantage of requiring only a minimal volume of blood for determination (0.05 ml whole blood) [59]. From the amount of surface markers studied, neutrophil CD11b and CD64 appear most promising for diagnosis of neonatal sepsis. CD11b expression increases considerably within a few minutes after the inflammatory cells come into contact with bacteria and endotoxins [68,69]. The sensitivity and specificity of CD11b for diagnosing early-onset neonatal sepsis are 86–100 and 100%, respectively [5,59]. CD64 has a sensitivity ranging between 78 and 96% and a NPV between 89 and 97% [70,71]. Though promising, estimation of cell surface markers is limited by the need for sophisticated equipment and the need to process blood samples rapidly before neutrophils die from apoptosis or the surface antigens are downregulated [72].

Despite the favorable claims by many studies, the high costs, the limited availability of specimens at the appropriate time, the complexity of the assay methods, the laboratory turnover time, and the test reliability all limit the clinical applicability of most diagnostic markers [59]. More importantly, the relatively small sample size in most studies and the lack of clear reference values for many markers still prohibit the use of many of them in clinical practice.

Currently, hematological indices including white blood cell count and absolute neutrophil count are frequently used to help assess the likelihood of infection in neonates that are symptomatic or at risk. The studies of both reference ranges and diagnostic accuracy report widely differing results [21,73,74,75]. In a current analysis on almost 70,000 symptomatic or at-risk neonates, Newman et al. [73] demonstrated that during the first 72 h of life white blood cell count and absolute neutrophil count are associated with an increased likelihood of infection only when they are low, so that the informative value increases with postnatal age. The area under the receiver operating characteristics curve for the white blood cell count and absolute neutrophil count determined within the first hour of life was 0.52 and 0.55, respectively, but increased significantly when determined after at least 4 h of life (0.87 and 0.85, respectively) [73].

An elevated immature to total neutrophil ratio is well associated with neonatal sepsis, but its diagnostic accuracy for early-onset infections shows again wide-ranging values [21]. Furthermore, its determination is subject to observer bias and thus of poor reproducibility.

At the moment, none of the described current diagnostic markers are sensitive and specific enough to influence the decision whether or not to withhold antimicrobial treatment independent of the clinical findings. Efforts were made to improve diagnostic accuracy by combining multiple markers in order to further enhance the diagnostic accuracy of these mediators in identifying infected cases.

CRP has been investigated in combination with a variety of ‘new’ infection markers including cytokines, surface markers, and other acute-phase reactants with promising results. Especially the combination with an early sensitive marker such as PCT, IL-6, IL-8, CD11b, and CD64 increases the sensitivity to values between 90 and 100% in most studies (see table 2).

Table 2

Diagnostic accuracy of CRP in combination with IL-6 (pg/ml), IL-8 (pg/ml), PCT (ng/ml), and CD64 (antibody-phycoerythrin molecules bound/cell)

Diagnostic accuracy of CRP in combination with IL-6 (pg/ml), IL-8 (pg/ml), PCT (ng/ml), and CD64 (antibody-phycoerythrin molecules bound/cell)
Diagnostic accuracy of CRP in combination with IL-6 (pg/ml), IL-8 (pg/ml), PCT (ng/ml), and CD64 (antibody-phycoerythrin molecules bound/cell)

What Is Already Known?

● The delayed induction of the hepatic synthesis of CRP during the inflammatory response to infection lowers its sensitivity during the early phases of sepsis. The performance of serial determinations 24–48 h after the onset of symptoms is recommended, as it clearly improves diagnostic accuracy.

● CRP is particularly useful for monitoring the response to treatment and for ruling out an infection. A repeated determination of CRP 24–48 h after the initiation of antibiotic therapy has been reported to carry a 99% negative predictive value in accurately identifying uninfected neonates, though nothing replaces a clinical impression and the gold standard (i.e. culture results).

● CRP values undergo a physiological 3-day rise after birth. This physiologic dynamics as well as certain maternal and perinatal factors may affect interpretation of what constitutes ‘normal’ CRP values in healthy neonates. Furthermore, some reports suggest noninfectious confounders such as meconium aspiration syndrome and perinatal maternal risk conditions may significantly elevate CRP values in symptomatic or at-risk neonates and thus confound interpretation of CRP values in the diagnosis of sepsis.

What Is New?

● A growing body of evidence suggests a link between gestational age and CRP kinetics with lower baseline CRP values and a lower CRP response to infection in preterm compared to term newborns.

● Still, data on noninfectious CRP elevations in otherwise healthy newborns as well as symptomatic and at-risk neonates demand further research in this topic before recommendations on the continuation or withdrawal of antibiotics in these infants can be given.

● Currently, the most used cutoff value is 10 mg/l irrespective of the gestational and postnatal age of the neonate. In view of the physiologic dynamics of CRP during the first days after birth and the influence of gestational age on its response to infection, it appears reasonable to reconsider this static cutoff value and evaluate the possible advantages of the introduction of dynamic reference values.

● CRP has the best diagnostic accuracy when combined with another infection marker that compensates for its diagnostic weakness and provides reliable sensitivity during the early phases of sepsis. Suitable markers include but are not limited to PCT, IL-6, and IL-8. Many further parameters may provide similar good results, but are not yet sufficiently examined to be applied in clinical practice.

CRP is one of the most widely available, most studied, and most used laboratory tests for neonatal bacterial infection, and despite the continuing emergence of new infection markers, it still plays a central role in the diagnosis of early-onset sepsis of the neonate. CRP has the advantage of being well characterized in numerous studies, and the extensive knowledge of its properties and limitations makes it safer compared to other, newer markers. Still, further research is needed on the topics of the influence of gestational age on CRP kinetics in infection, noninfectious confounders, and the evaluation of dynamic and gestational age-dependent reference values.

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