In capillary blood collection, wiping the first drop is used to prevent interference from which substance?

• A. Alcohol residue
• B. Air bubbles
• C. Hemoglobin degradation
• D. Thromboplastin contamination

Answer: (D)

The main idea is to avoid introducing tissue-derived substances that can alter test results. When the skin is pricked for capillary collection, damaged cells release thromboplastin (tissue factor). If the first drop of blood is allowed to remain, this thromboplastin can contaminate the sample and prematurely activate coagulation or otherwise skew results, especially for tests that depend on the coagulation pathway. Wiping away that initial drop removes this potential contaminant, yielding a sample that better reflects circulating blood rather than tissue factors at the puncture site. Some other potential interferences, like alcohol residue, are addressed by cleaning, but the specific concern here is tissue-factor contamination from the puncture.