PTINR

Prothrombin Time International Normalized Ratio (PT/INR) is a standardized test that assesses the blood's tendency to clot. It derives from the prothrombin time (PT) test, which measures how long plasma takes to clot after adding tissue factor and calcium. The INR adjusts the PT result to account for variability in thromboplastin reagents between laboratories.

The INR is calculated using the formula INR = (PTpatient / PTnormal) ^ISI, where PTpatient is the clotting time in seconds, PTnormal is a laboratory reference value, and ISI is the international sensitivity index supplied by the reagent manufacturer. The goal is to produce a numeric value that is comparable across labs.

PT/INR is used primarily to monitor oral anticoagulation with warfarin or related drugs. A normal INR for people not on anticoagulation is about 0.9 to 1.2. Therapeutic ranges vary by indication and valve status; common targets are 2.0–3.0 for many atrial fibrillation or venous thromboembolism patients, and 2.5–3.5 for certain mechanical heart valves. Some guidelines may differ slightly.

Sampling is usually done by venous blood draw; results can be reported in INR or raw PT seconds, with INR being standard. Point‑of‑care devices also provide INR. DOACs (direct oral anticoagulants) generally do not require INR monitoring.

Several factors can alter INR, including liver disease, vitamin K intake or supplementation, antibiotics that disrupt gut flora, diet, adherence, and drug interactions. Reagent quality, instrument calibration, and time from sample collection can also affect results. INR interpretation should consider the clinical context and laboratory reference ranges.