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Specimen integrity is central to ASCP objectives. Bilirubin is light-sensitive; exposure to ambient or UV light causes photodegradation, yielding falsely low values. To preserve accuracy, the tube should be wrapped in foil or placed in an amber container immediately after collection and kept protected through transport and processing. Other light-sensitive analytes (e.g., carotene, porphyrins) may have similar requirements, but bilirubin is the most commonly tested. Beyond light protection, preanalytical variables such as prolonged tourniquet time, hemolysis, and delayed centrifugation can affect results. For serum, allow complete clotting (typically ~30 minutes unless using a rapid serum tube) before centrifugation; for plasma, invert per tube instructions to ensure proper anticoagulant mixing. Temperature control also matters: some analytes (e.g., ammonia, lactate) require chilling, while others should be kept at room temperature until processing. Correct labeling, timely transport, and adherence to specific test handling instructions (often found in the laboratory’s test directory) help prevent errors. By knowing which specimens require light protection, temperature control, or prompt separation, phlebotomists uphold analytical validity and patient safety.