Comparison of emergency department HIV testing data with visit or patient as the unit of analysis

Outcomes in an episodic care setting like an emergency department (ED) are traditionally evaluated in comparison with the number of visits as opposed to the number of unique patients, although patients commonly present to the ED multiple times. We examined the differences in HIV screening programme outcomes that would occur if the analysis were conducted at the patient-level, rather than the traditional visit-level. We hypothesized that while our ED-based HIV screening programme does test some patients repeatedly, the primary programme outcome of percent positive is not substantially altered by the unit of analysis.

We reviewed the clinical database of an ED HIV screening programme at a large, urban, teaching hospital in the United States from 2003-2007. Data were analyzed descriptively. The main outcome measure was the rate of positive test results computed with either the visit or the patient as the unit of analysis.

HIV testing was provided at 9629 visits, representing 8450 unique patients. For patient-level analysis, the proportion of patients found to be positive was 0.91%. For visit-level analysis, the proportion of tests with positive results was 0.83%. Of the 910 patients with repeat testing, 7 (0.77%) were identified as positive at a repeat test. The median time between tests was 383 days (range 1-1742).

Results changed little regardless of whether unique patients or unique visits were used as the unit of analysis. Any differences in positive rates were mitigated by the contribution of repeat testing to the identification of newly infected patients. Given these findings, and the difficulty of tracking repeat testing over time, visit-level analysis are appropriate for comparing programme outcomes when detailed modeling of epidemiology, cost, and/or outcomes is not required.