Five screening patterns, not five diagnoses

PT measures a clot-based endpoint that depends on the tissue-factor (extrinsic) pathway and the common pathway. aPTT measures a clot-based endpoint that depends on the intrinsic pathway and the common pathway. Platelet count is a cell count, not a function test. Fibrinogen (Clauss method, when the local method is declared) measures the substrate that both pathways ultimately convert to fibrin. Held together with a bleeding history, these four numbers sort into a small set of recognizable patterns rather than into a specific disease name.

Five patterns organize almost every initial case: an isolated low platelet count, an isolated prolonged PT, an isolated prolonged aPTT, a combined PT-and-aPTT prolongation, and a fully normal screen paired with a bleeding history that still argues for a hemostatic problem. Each pattern narrows a differential; none of them, alone, names a disorder. A normal platelet count rules out thrombocytopenia as the explanation but says nothing about whether the platelets that are present work correctly. A normal PT and aPTT do not exclude von Willebrand disease (VWD), a qualitative platelet disorder, factor XIII deficiency, or a fibrinolytic disorder, because none of those is reliably detected by the conventional screen.

The pattern-recognition sequence follows the order a bench scientist actually works it: confirm the specimen and medications are not the explanation first, gather the bleeding history, read the screen as a pattern rather than four separate numbers, then match the pattern to its differential family before choosing the next test.

Read PT, aPTT, platelet count, and fibrinogen together as one pattern, and treat every pattern, including a fully normal one, as a starting point rather than an answer.