Your call: three branched cases

For each branched case, choose the safest next step from the evidence available, then revise that choice when additional evidence changes the differential.

Case 1, isolated aPTT with an unknown anticoagulant: the aPTT is prolonged, the PT is normal, and the specimen was drawn from a line with no documented clearance. Your first decision is what to check before ordering anything advanced. New evidence: the line draw cannot be confirmed clean. The safest next step is a peripheral recollection before a mixing study, because a line-draw contamination can mimic a coagulopathy and a mixing study run on a contaminated specimen wastes time and plasma while risking a wrong localization.

Case 5, an inhibitor workup with a nonparallel factor VIII assay: the one-stage factor VIII activity comes back low, but the dilution curve is nonparallel. Your first decision is what that nonparallel pattern proves. New evidence: repeat dilutions confirm the nonparallel behavior is reproducible. Nonparallel dilutions raise concern for interference or an inhibitor; they do not, by themselves, identify a specific inhibitor, so inspect the dilution curve and, where validated, run a chromogenic factor VIII assay on the same specimen. Agreement supports that the low activity is reproducible across methods but does not name its cause, because a true inhibitor can lower both results. Disagreement points toward assay-specific interference rather than a settled diagnosis. Either outcome still calls for a targeted interference workup, not a same-day inhibitor titer released as definitive.

Case 6, an LA workup during anticoagulation: the patient is on a DOAC and needs an LA panel that cannot be deferred. Your first decision is what has to be resolved before the mix or the LA screen is interpreted, not just what the eventual report should say. New evidence: the prescriber confirms the drug cannot be safely interrupted. The defensible action is to document the DOAC, its last dose, and the patient's renal function, then apply the assay system's validated DOAC-mitigation step before reading a noncorrecting mix or an LA screen as positive, because a DOAC still circulating in the specimen can by itself keep a 1:1 mix from correcting and mimic an inhibitor pattern. With the drug's contribution addressed as far as the assay system allows, run the validated screen-mix-confirm panel across the assay systems named by the laboratory's procedure, and state the anticoagulant exposure and interpretation limits in the report and any verbal communication rather than issuing an unqualified positive or negative LA result or reading noncorrection itself as inhibitor localization.

Decision economy runs through all three cases: the next test is the one that most separates the leading explanations and changes the safe laboratory action, not a broad panel ordered out of habit. Ordering every possible confirmatory test at once uses specimen volume and turnaround time without adding discriminating information.

Hold your first answer up against each new fact and be willing to change the ranking, the next test, or the wording of the report as new evidence arrives.