A bilirubin result at 10 AM was 12 mg/dL and retested at 3 PM was 8 mg/dL. The most likely explanation is?

• A. The reagent has deteriorated
• B. The sample was exposed to light
• C. A calculation error in the first assay
• D. The sample was not refrigerated

Answer: (B)

Bilirubin is highly sensitive to light. When a sample is exposed to light, bilirubin undergoes photodegradation (photobleaching), which lowers the amount detected by the assay. In this scenario, the bilirubin value dropped from 12 mg/dL to 8 mg/dL over the afternoon while the sample was likely sitting in light, causing the second measurement to be falsely low. Protecting samples from light (using amber tubes, dark trays, or wrapping) prevents this artifact.

Reagent deterioration would more likely cause inconsistent results across a batch rather than a single sample to progressively decrease over time. A calculation error in the first assay would not explain a believable, time-dependent drop in the second measurement. Not refrigeration could affect stability some, but bilirubin is relatively stable with proper refrigeration; light exposure remains the classic explanation for such a pattern.