title: ICD-9 CM code 333.1 as an identifier of patients with essential tremor: a study of the positive predictive value of this code.
abstract: OBJECTIVE
  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).
  
  
  CONCLUSIONS
  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.
authors.affiliation: Gertrude H. Sergievsky Center, College of Physicians and Surgeons, Columbia University, New York, USA. EDL2@columbia.edu
authors.forename: Elan D
authors.initials: ED
authors.lastname: Louis
authors.forename: Lakeisha M
authors.initials: LM
authors.lastname: Applegate
authors.forename: Eileen
authors.initials: E
authors.lastname: Rios
mesh_terms: D020329:Essential Tremor / Q000145:classification* / Q000175:diagnosis*; D006801:Humans; D038801:International Classification of Diseases / Q000592:standards*; D008499:Medical Records / Q000592:standards; D011237:Predictive Value of Tests; D012189:Retrospective Studies