The thrombectomy analysis contains its own correction, and it is the most instructive part of the paper.
Acute kidney injury appeared twice as common with intensive blood pressure management. But intensive management means more frequent monitoring, and acute kidney injury is defined by a change in creatinine — an outcome you can only detect if you measure it. When the authors ran a sensitivity analysis restricted to patients with equal creatinine ascertainment, the 48-hour association fell from significant to an odds ratio of 4.20 with an interval crossing 1.0.
This is ascertainment bias, and it is endemic to any outcome defined by a test result rather than by an event. A patient who dies is counted whether or not anyone was looking; a patient whose creatinine rises 30 micromoles is counted only if someone drew the blood.
The habit: for any complication defined by a laboratory value, ask whether the groups were tested equally often. If they were not, the difference in rates is partly a difference in looking — and the direction of that bias always favours finding more disease in the more closely watched arm.
- Outcomes defined by test results require equal testing to compare fairly
- More intensive treatment usually means more intensive monitoring
- The bias always inflates the complication rate in the watched group
- Look for a sensitivity analysis restricted to equal ascertainment
- Hard outcomes — death, dialysis — are far less vulnerable to this
The statistics, in plain English
Ascertainment bias differs from confounding: it is not that the groups differ in risk, but that they differ in the chance of a real event being recorded. Adjustment cannot fix it, because the missing events are absent from the data entirely. Only restricting the analysis to equally monitored patients, as done here, addresses it — and it usually shrinks the effect.
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