APTT stands for Activated Partial Thromboplastin Time, which is a blood test used to evaluate a person's blood clotting or coagulation system. It measures the time it takes for blood to clot in response to certain activators in a laboratory setting. APTT is often used to monitor the effectiveness of blood-thinning medications like heparin and to diagnose and assess bleeding disorders such as hemophilia. It is typically expressed in seconds and can provide valuable information about a person's risk of excessive bleeding or excessive clotting. Abnormal APTT results can indicate various medical conditions and may require further investigation by a healthcare provider.