FaridabadBy Dr B.C. Gupta, MD · 2026-10-15
Ask the Pathologist - MCQ of the Week. A short question to test how well you know common laboratory tests, with a plain-English explanation from Dr B.C. Gupta, MD (Pathology).
Q. Which test measures how long it takes for blood to form a clot?
- A. Prothrombin time with INR
- B. Vitamin D
- C. Serum calcium
- D. Urine routine examination
Answer: Prothrombin time with INR
Prothrombin time, reported with the INR, measures how long a blood sample takes to form a clot. It checks the clotting proteins that the liver makes with the help of vitamin K. The INR expresses the result as a ratio against a standard, so the number means the same thing in any laboratory, which is why it is used to monitor blood thinners such as warfarin.
A closely related test is the APTT, which checks a different part of the clotting system and is often done at the same time. The two tests together give a fuller picture than either alone. For most people on warfarin the target INR is usually between 2 and 3, but the exact target is set by the doctor.
Vitamin D, serum calcium and a urine routine examination do not measure clotting. Vitamin D and calcium relate to bone and mineral health, and a urine routine examination checks the urine for infection, blood and other findings. If your doctor has asked for a PT/INR or a clotting test, call the centre to confirm preparation and timing before you visit.
