What the screen supports, what it does not, and what depends on your lab

The screen supports pattern recognition: a low platelet count supports thrombocytopenia as an explanation, an isolated prolonged PT or aPTT supports a specific short list of causes, and a combined prolongation supports a broader consumptive or synthetic differential. None of these results, alone or together, establishes a final diagnosis; that requires the targeted follow-up study chosen for the pattern, plus clinical correlation that this screen cannot provide.

The screen does not support ruling out VWD, a qualitative platelet disorder, factor XIII deficiency, or a fibrinolytic disorder from a normal PT, aPTT, platelet count, and fibrinogen. It also does not support ruling out mild hemophilia A or B from a normal aPTT alone. Every reference interval, cutoff, mixing-study correction criterion, D-dimer category, and critical value used on your bench is set and verified locally, by your laboratory's reagent, analyzer, and citrate concentration; none of the numbers discussed here should be carried forward as a portable number.

When a bleeding history is real and the screen is normal, the next step is a targeted VWF and factor VIII work-up, not a closed case.