Background/Aims: Health outcomes research often uses administrative databases. Patients with the diseases of interest are identified using International Classification of Diseases (ICD-9 CM) codes. The utility of the code for essential tremor (ET), 333.1, remains untested. We determined the positive predictive value (PPV) of the code 333.1. Methods: Patients with the ICD-9 CM code 333.1 were identified from billing records at the Neurological Institute of New York. Their medical records were reviewed to determine whether they met Consensus Criteria for ET. Results: Of 964 patients who carried the code 333.1, only 472 met diagnostic criteria for ET (i.e. PPV = 49.0%). The additional use of ICD 9-CM codes for parkinsonism and dystonia (as exclusionary criteria) only marginally improved this value (57.8%). Common diagnoses among the false positives were Parkinson’s disease, dystonia, enhanced physiological tremor, drug-induced tremor, orthostatic tremor, and psychogenic tremor. Patients seen by general neurologists (vs. movement disorder specialists) were half as likely to meet diagnostic criteria for ET (34.6 vs. 51.9%, OR 0.50, 95% CI 0.23–0.70, p < 0.001). Conclusion: The code 333.1 performed poorly when attempting to identify ET cases. Given the very high prevalence of ET, a unique diagnostic code would seem to be in order.

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