Working the Guided Case

PT, INR, TT, and Clauss fibrinogen all sit inside their local intervals. Only the aPTT is prolonged, at 68.4 seconds against 24.0 to 36.0 seconds. Because TT and fibrinogen are both normal, a broad problem with final fibrin formation or polymerization is less likely; whatever is happening, it is not showing up as a fibrinogen-to-fibrin defect.

Because the PT is normal, a broad common-pathway or factor VII problem large enough to show up on this reagent system is also less likely. That leaves the isolated aPTT prolongation pointing broadly toward the contact/intrinsic factors XII, XI, IX, or VIII, toward heparin or another anticoagulant effect, or toward an inhibitor, none of which this pattern can distinguish on its own.

A heparin-calibrated anti-Xa assay reports residual factor Xa activity against a heparin calibration curve. The 0.58 IU/mL result is compatible with anti-Xa activity, but it does not identify heparin or exclude a factor Xa inhibitor. No local therapeutic interval was supplied, so the result cannot be read against a therapeutic range here. Review the medication list and draw source, including a possible line draw, before attempting identification.

Resolve anticoagulant interference first. If mixing is appropriate, use the laboratory's validated correction method and cutoff. If factor activity is appropriate, interpret one-stage and chromogenic results as method-dependent.

An isolated aPTT prolongation with a normal PT, TT, and fibrinogen narrows the field, it does not close the case.