A heparin lock, a normal INR, and an aPTT over 100 seconds

A citrate tube is drawn at 09:10 from a central line documented as heparin locked. The order is PT, aPTT, thrombin time (TT), and unfractionated heparin (UFH) anti-Xa. No anticoagulant history accompanies the specimen. The specimen is not hemolyzed or clotted and is adequately filled, so a preanalytic rejection is not in play.

The panel comes back mixed. PT is 12.1 seconds against a local interval of 10.0 to 13.0, and the international normalized ratio (INR) is 1.0 against 0.9 to 1.1, both squarely normal. aPTT is 118 seconds against 25 to 35, markedly prolonged. TT is greater than 180 seconds against 14 to 21, with no clot endpoint reached. Heparin-calibrated anti-Xa is 1.34 IU/mL against 0.00 to 0.10, far above the untreated interval.

A normal PT and INR next to a wildly abnormal aPTT, TT, and anti-Xa is not a contradiction to explain away. It is a pattern: a marked global heparin-sensitive prolongation with a spared vitamin K antagonist (VKA) marker, on a specimen collected from a heparin-exposed line with no documented infusion, dose, or timing. Before this panel can be interpreted as a patient's anticoagulant status, the collection itself has to be evaluated.

Each anticoagulant class prolongs a different set of tests for a mechanistic reason, only certain tests actually measure a given drug, and when the specimen history itself is a candidate explanation, that has to be worked through too. For every case like this one: which anticoagulant is present, which tests can it affect, and which result can be interpreted safely.