Positive Predictive Values of ICD-9 Codes to Identify Patients With Stroke or TIA | Page 2

Published Online: February 26, 2014
Kari L. Olson, BSc(Pharm), PharmD; Michele D. Wood, PharmD; Thomas Delate, PhD; Lisa J. Lash, PharmD; Jon Rasmussen, PharmD; Anne M. Denham, PharmD; and John A. Merenich, MD
Prescriptions for antiplatelet medications (ie, ticlopidine [Ticlid], clopidogrel [Plavix], dipyridamole [Persantine], and dipyridamole/ ASA [Aggrenox]), use of imaging (CT and/or MRI scans), and outpatient neurology department visits within 180 days of initial cerebrovascular ICD-9 code(s) were identified using queries of KPCO-integrated electronic databases. These stroke-related health service use indicators were used in conjunction with the study ICD-9 codes to determine if the PPVs of administrative data to identify strokes could be improved.

Data Analysis

Positive predictive values for each ICD-9 code were determined by dividing the number of confirmed stroke events by the total number of events recorded in the administrative data for the specified code. The primary study outcome was the PPV with 95% confidence intervals (CIs) for correctly identifying confirmed cerebral events.19 PPVs were calculated for each ICD-9 code in the setting where the code was recorded (ie, inpatient setting only, outpatient setting only, or the same code recorded in both settings). Secondary outcomes included assessments of PPV for identifying confirmed hemorrhagic and ischemic strokes, respectively for each hemorrhagic (430, 431, 432) and ischemic (433, 434, 435, 436, 437, 438, V12.54) code. Positive predictive values for these outcomes were determined by dividing the number of specific confirmed events (ischemic or hemorrhagic) by the total number of events recorded in the administrative data for the specified code. Subanalyses were performed to determine the robustness of PPV estimates under various scenarios.

The impact of using study ICD-9 codes in conjunction with health service indicators was determined by calculating PPVs for each ICD-9 code individually and in combination with various health service indicators. To accomplish this, only patients who were exposed to the health service indicator and had the ICD-9 code diagnosis were used to calculate the PPV. To assess the impact of ICD-9 code position (primary or secondary), secondary position inpatient codes and patients with only a secondary position inpatient code were removed and PPVs recalculated. To assess the impact of including TIA, all patients with confirmed TIA were removed and PPVs recalculated. To assess the impact of the ICD-9 code 436 not being inclusive of stroke, because the code excludes “cerebrovascular accident (CVA) NOS, Stroke” as of October 1, 2004, the PPVs of code 436 recorded before and on/after this date were recalculated. To assess the impact of the use of codes with specific “infarction” terms, codes 433.XX and 434.XX were categorized individually by codes that contain the infarction term (ie, 433.X1 and 434.X1) or not and the PPVs were recalculated. Positive predictive values, were determined with SAS version 9.1.3 (SAS Institute Inc, Cary, North Carolina) using Proc Freq with weighting by the count of patients having a specific diagnosis and the exact binomial function to determine 95% CIs.

Patient characteristics were reported as means with standard deviations for interval-level characteristics. These characteristics were assessed for distribution normality and appropriate tests (eg, t test, rank-sum test) were used to assess differences between groups. To assess differences in proportions between groups on dichotomous characteristics, Pearson’s x2 test of association was utilized. A 2-sided alpha level was set at <.05.


A total of 4689 patients with 10,376 unique study administrative ICD-9 codes were reviewed. Of these, 2785 (59.4%) patients had a cerebral event confirmed by EMR review. The majority of patients had ICD-9 codes from the outpatient setting (82.6%) while 1.3% and 16.1% were from inpatient and both outpatient and inpatient settings, respectively (Table 1). The most commonly identified cerebral event types were non-cardioembolic strokes (34.8%) and TIAs (31.1%). Cerebral event type was unknown in 15.4% of cases. Patients with confirmed cerebral events had a higher mean count of unique ICD-9 codes, were slightly older, more likely to have purchased a prescription antiplatelet drug, and more likely to have had CT or MRI imaging. Positive predictive values for “intracerebral hemorrhage”( 431), “acute but ill-defined cerebrovascular disease” (436), and “personal history of stroke” (V12.54) were greater than 90% when recorded in both the inpatient and outpatient settings but identified small numbers of patients; thus, associated 95% CIs were wide (Table 2). “Occlusion of cerebral arteries” (434) recorded in either the inpatient-only or in both the inpatient and outpatient settings also achieved PPVs greater than 90% and identified large numbers of patients. Codes 434 and V12.54 in the outpatient-only setting identified the most patients and had reasonably high PPVs (both 89%). “Other and unspecified intracranial hemorrhage” (432) and “other and ill-defined cerebrovascular disease” (437) performed poorly regardless of setting (PPVs <50%). Overall, codes recorded in both the inpatient and outpatient settings yielded the highest PPVs but identified fewer patients compared with those recorded only in the inpatient or outpatient settings. Hemorrhagic stroke codes identified fewer patients than ischemic stroke codes (Table 3). Hemorrhagic stroke codes recorded only in the inpatient setting had higher PPVs than outpatient-only codes. Overall, ischemic codes tended to have higher PPVs than hemorrhagic codes (Table 4). Approximately 15% of code 436 patients had their code recorded on/after October 1, 2004, and the vast majority of these (99%) were in the outpatient setting. Nevertheless, there was no appreciable change in PPV before and after 436’s coding modification. Code 433.X1 with specific mention of infarction did increase the PPV appreciably but only approximately 8% of patients with a 433.XX code had a 433.X1 code. Code 434.X1 with specific mention of infarction did not alter the PPV appreciably over presence of the code 434.XX (Table 2).

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Issue: February 2014
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