Strategy for reliable identification of ischaemic stroke, thrombolytics and thrombectomy in large administrative databases

Patient and procedure identification

We identified all ED and hospital discharges from acute care hospitals for ischaemic stroke from 2010 to 2017 based on primary ICD-9-CM discharge codes (433 .xx excluding 433.10, 434 .xx and 436 for discharges from 2010 through the third quarter of 2015), or primary ICD-10-CM codes (I63 for discharges from the fourth quarter of 2015-2017), or MS-DRG codes (061, 062, 063).1-4 8 11 Hospital discharges identified any inpatient hospital stay, whereas ED discharges were used to identify ED records of patients who were subsequently transferred to another hospital for inpatient stay. In addition, we also obtained data on MS-DRGs codes 064, 065 and 066 which may be used for patients who had an ischaemic or haemorrhagic stroke, and 023 and 024 codes which may be used for patients who had an ischaemic stroke who receive EVT, or for other procedures not related to ischaemic stroke. When any of these additional MS-DRGs (064, 065, 066, 023 and 024) were used, we required one of the specified ICD-9-CM or ICD-10-CM codes for ischaemic stroke to be present also to identify the subject as a case of interest.

We identified patients transferred between hospitals using an established strategy.13 14 After identifying all ischaemic stroke hospitalisations, we used a unique patient identifier to look backward in ED and inpatient data to identify any earlier records. The earlier record could have any length of stay, provided that the discharge date was on the preceding or the same day as the index admission. If this earlier record had a discharge disposition consistent with transfer, and a discharge date that was the preceding or the same day as the index hospital admission date, then these records were linked to establish an ED-to-inpatient or an inpatient-to-inpatient transfer. We did not require any particular ICD or MS-DRG discharge code to be associated with the initial visit, recognising that, especially for patients transferred from an ED, a final diagnosis is often not yet established. The use of another code (eg, for weakness or headache) would not preclude a transfer for stroke as long as the final discharge diagnosis from the second hospital met our case definition of ischaemic stroke.

Using established methods and approaches,6 15 we identified patients receiving thrombolytic based on the presence of any one of the following: ICD-9-PCS code (9910), a secondary ICD-9-CM code (V4588), ICD-10-PCS code (3E03317), a secondary ICD-10-CM code (Z9282), CPT codes (37195, 37201, 37202) or MS-DRG codes (061, 062, 063 alone; or 065 with a corresponding ICD-9 or ICD-10 code indicating alteplase receipt). These codes were not required to be in the primary position.

We identified patients treated with EVT based on the presence of any one of the following: ICD-9-PCS code (3974, 1753, 1754), ICD-10-PCS code (03CG3ZZ, 03CH3ZZ, 03CJ3ZZ, 03CK3ZZ, 03CL3ZZ, 03CM3ZZ, 03CN3ZZ, 03CP3ZZ, 03CQ3ZZ) or MS-DRG (023, 024).6 7 MS-DRG codes 023 and 024 may also be used for craniotomy or device implantation procedures. Therefore, when 023 and 024 were present, we only identified EVT among observations with a primary ICD-9/ICD-10 diagnosis of ischaemic stroke, and who did not have any ICD-9-PCS/ICD-10-PCS codes for craniotomy, craniectomy or ventriculostomy (online supplemental table IV).16-19 These codes were not required to be in the primary position. We did not use CPT codes for EVT because (consistent with national coding standards) our data only used CPT codes for ED patients, and all EVT-treated patients were treated as inpatient hospitalisations rather than ED patients.

We examined patient age, sex, race/ethnicity, expected payer and urban/rural location, overall and for each group of patients based on identification strategy. Urban versus rural location was based on county-level urban influence codes in 2013.20

We categorised each ischaemic stroke observation based on whether it was identifiable by ICD code only, by MS-DRG only or by both. Within each group, we determined the proportions of transfer, thrombolytic-treated and EVT-treated patients identified.

Among patients identified by ICD-9 or ICD-10 code only (ie, without one of our designated MS-DRG codes), we identified and reviewed the 10 most frequent MS-DRG codes used. Likewise, among patients identified by MS-DRG code (061, 062, 063) without a designated ICD-9 or ICD-10 code, we identified and reviewed the 10 most frequent ICD-9 or ICD-10 codes used.

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