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Coagulation testing depends on a precise 9:1 blood-to-citrate ratio. Underfilling increases the relative citrate concentration, chelating proportionally more calcium during testing and artifactually prolonging clot-based assays (PT, aPTT, fibrinogen) to variable degrees. The correct response is to reject and recollect a properly filled tube. Never “top off” from another tube (Option A); this causes additive carryover and contamination. Accepting half-full tubes (Option B) is unacceptable; even modest short draws can bias results, particularly in pediatric microtubes. Selectively reporting PT (Option D) still risks error because both assays rely on correct ratio. Prevent underfill by choosing an appropriate vein/gauge, using a discard with butterfly sets before citrate tubes, and ensuring the tube vacuum is intact. Document recollection and educate staff on the criticality of fill volumes—a frequently tested preanalytical point.