Why is the first drop of blood wiped away during capillary collection?

• A. Alcohol residue
• B. Thromboplastin contamination
• C. It is too viscous
• D. It clots immediately

Answer: (B)

The main idea is to prevent contamination from damaged tissue when collecting capillary blood. When the skin is punctured, the first drop can carry tissue fluids and factors released from skin cells, including thromboplastin (a tissue factor). If that drop is collected, it can promote clotting or otherwise skew test results, especially for tests sensitive to coagulation or to contaminating substances. Wiping away the first drop helps ensure the sample reflects circulating blood rather than mixtures from the puncture site. The other choices don’t explain the standard practice: alcohol residue is handled by letting the antiseptic dry before puncture, the drop’s viscosity isn’t the issue, and immediate clotting isn’t the typical reason for discarding the first drop.