The RST clotting time is limited to how many minutes?

• A. 1 minute
• B. 5 minutes
• C. 10 minutes
• D. 15 minutes

Answer: (B)

Clotting time tests measure how quickly blood forms a clot under standardized conditions. The RST clotting time is designed as a rapid, qualitative screen with a fixed observation window, set at five minutes. If a clot appears within five minutes, coagulation is considered normal for this test. If no clot is seen by five minutes, the result is deemed prolonged, suggesting a coagulation defect, the influence of anticoagulants, or the presence of inhibitors. The five-minute limit helps keep the result consistent between runs and different operators; extending the window would introduce more variability due to handling, temperature changes, and reagent stability. In short, five minutes is the practical cutoff that defines a normal versus prolonged clotting time in this assay.