Positive Predictive Values of ICD-9 Codes to Identify Patients With Stroke or TIA | Page 3

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
In general, inclusion of health service indicators and exclusion of secondary inpatient diagnoses and TIA patients identified fewer patients, widened 95% CIs and did not change the PPVs substantially. Overall, inclusion of diagnostic imaging did not change PPV estimates, nor did inclusion of neurology visits. Inclusion of prescription antiplatelet exposure slightly improved PPVs (mean difference in PPVs across care settings = 1.5 [± 4.2], median difference in PPVs across care settings = 1.0). However, for codes 434 and V12.54, inclusion of prescription antiplatelet exposure resulted in more marked improvement in PPV (mean = 8.6 [± 9.7], median = 4). Removing secondary position inpatient codes did not affect the PPVs appreciably (mean = –0.5 [± 3.0] and median = 0), but removing patients with a confirmed TIA (code 435) slightly reduced PPVs overall (mean = –2.0 [± 2.5] and median = –1).


Disease registries provide opportunities for health systems to improve management of patients with chronic disease states. Initial patient identification using coded administrative data is an important part of developing a validated patient registry, particularly if the codes have high PPV. Using a standardized abstraction tool adapted from the Rochester Minnesota Stroke study form,18 we found only 60% of patients identified from administrative data using cerebrovascular ICD-9 codes had a confirmed cerebral event. We found that the settings where ICD-9 codes were recorded influenced both the accuracy of diagnosis and yield of identified cases. Codes recorded in both inpatient and outpatient settings had higher absolute PPVs, but identified fewer patients than codes recorded in only 1 of these settings. Attempts to improve the accuracy of ICD-9 codes through various combinations with health services indicators produced, at best, only moderate improvements, with the exception of combining purchases of prescription antiplatelets with codes 434 and V12.54. The incorporation of the setting where the ICD-9 codes were recorded and using combinations health service indicators are unique aspects of our study. Nevertheless, in most cases, our PPV estimates were associated with wide 95% CIs despite a relatively large sample size suggesting that administratively coded data elements may lack sufficient accuracy to be relied on without confirmation of cerebral events via medical record review.

Several inpatient studies have evaluated the accuracy of ICD-9 codes for identifying ischemic stroke by assessing the sensitivity, specificity, and/or PPV of ICD-9 diagnosis codes 430 to 438.4-7,9-16 These studies also demonstrated less than optimal accuracy of ICD-9 codes in identifying confirmed stroke patients. One study reported a PPV of only 47% for ICD-9 codes between 430 and 438 for correctly identifying incident stroke events.5 Additionally, several studies have revealed that registries derived from hospital discharge codes overestimate stroke.5,6,16 While our use of ICD-9 codes recorded in inpatient and/or outpatient settings appears to modestly improve the accuracy of identifying confirmed cerebral events, the use of ICD-9 codes alone appears to lead to a high percentage of false-positive diagnoses, such that about 40% of events identified with these commonly used ICD-9 codes are not confirmed cerebral events.6

Benesch and colleagues found that limiting inpatient ICD-9 codes to those listed in the primary discharge position increased stroke PPV.10 We hoped to increase event capture by using ICD-9 codes recorded in the primary or secondary discharge positions. Nevertheless, we also performed a subanalysis using only the primary position and found that removing secondary position codes did not decrease our inpatient PPVs. Our results may have differed from their study since we had relatively low rates of false positive strokes.

We reported PPV estimates for codes recorded only in the outpatient setting, capturing patients who may or may not have been hospitalized for treatment prior to enrolling in our health plan. We found that considerably more patients were identified in the outpatient setting. Similar to prior studies, we found that ICD-9 codes 434 and 436 had high PPVs for patients with confirmed ischemic stroke.10,11 We were able to show that the PPVs for these codes slightly improved when the codes were recorded in both the inpatient and outpatient settings (97% and 93%, respectively). Since October 1, 2004, when 436 coding changes were implemented, the number of patients with this code decreased considerably, making the utility of this code to identify patients with stroke less robust.

We assessed ICD-9 code 435 (TIA) because treatment guidelines are similar for patients who experienced an ischemic stroke and TIA is an important risk factor for stroke, with 90-day risks reported as high as 17%.2 Interestingly, we found that code 435 was one of the most prevalent codes, capturing 1329 patients who had their cerebral event confirmed, and its PPVs were relatively high across settings (>84%) indicating potential utility for this code if identification of patients with probable TIA is desirable.

Historically, ICD-9 codes for hemorrhagic stroke have reported higher PPVs than ischemic stroke codes.14,15 However, we found that, overall, hemorrhagic stroke codes had lower PPVs than ischemic stroke codes, perhaps because there were fewer patients with hemorrhagic stroke codes. We did find that inpatient hemorrhagic codes had higher PPVs than outpatient hemorrhagic codes suggesting that inpatient hemorrhagic events are coded more accurately.

It is not surprising that ICD-9 codes 432 “other and nonspecified intracranial hemorrhage” and 437 “other and illdefined cerebrovascular disease” performed poorly based on their definitions. For example 437 includes a variety of conditions under the umbrella of “ill defined,” including cerebral atherosclerosis, chronic cerebral ischemia, hypertensive encephalopathy, non-ruptured cerebral aneurysm, moyamoya disease, nonpyogenic thrombosis of intracranial venous sinus, and transient global amnesia.10

Overall, the inclusion of health service indicators such as diagnostic imaging and neurology visits did not change appreciably the PPV estimates, nor did evaluating code 434.X1 with specific infarction terms. Code 433.X1 with specific mention of infarction did increase the PPV appreciably but identified only a small number of patients. Inclusion of prescription antiplatelet exposure slightly improved PPVs. We identified no other studies that employed health service indicators to assess their effects on PPVs.

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Issue: February 2014
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