What does the partial thromboplastin time test measure?

The partial thromboplastin time test measures how long it takes blood to clot to help determine bleeding problems, explains WebMD. Blood is drawn from the veins into an air-tight container, with chemicals added to the sample to gauge how long it takes the blood to clot, states MedlinePlus.

While normal value ranges may vary slightly among different labs, clotting should typically occur within 25 to 35 seconds, according to MedlinePlus. The time it takes blood to clot may be affected by blood-thinning medicines, inhibitors, and low levels or the absence of blood clotting factors -- substances in the blood that help stop bleeding when blood vessels are damaged, explains WebMD.