Examkin

Question

Difficulty: Very hardOrdering Laboratory Tests

A 34-year-old woman is evaluated in the preoperative clinic prior to a scheduled elective cholecystectomy for symptomatic cholelithiasis. She has no significant past medical history and takes no medications. She has never undergone surgery, but she reports having two uncomplicated vaginal deliveries and no history of excessive bleeding following previous dental extractions.

Preoperative laboratory results show:

Hemoglobin: 13.5 g/dL
Platelet count: 245,000/mm³
Prothrombin time (PT): 12 seconds (normal: 11–14 sec)
Activated partial thromboplastin time (aPTT): 62 seconds (normal: 25–35 sec)
Liver function tests are within normal limits.

Which of the following is the most appropriate next step in laboratory evaluation?

  1. 1:1 Mixing study with normal plasmaAnswer
  2. B
    Factor VIII, IX, and XI activity assays
  3. C
    Dilute Russell viper venom time (dRVVT)
  4. D
    Platelet function analyzer (PFA-100) testing
  5. E
    Von Willebrand factor antigen and ristocetin cofactor activity

Answer

The most appropriate next step is to perform a 1:1 mixing study with normal plasma to determine if the prolonged aPTT is due to a factor deficiency or an inhibitor.
The correct next step is a 1:1 mixing study. This patient presents with an isolated prolongation of the activated partial thromboplastin time (aPTT) and an entirely negative clinical bleeding history. The differential diagnosis for an isolated aPTT prolongation includes intrinsic factor deficiencies (Factors VIII, IX, XI, XII) or the presence of a circulating inhibitor (most commonly a lupus anticoagulant). A 1:1 mixing study combines the patient's plasma with normal pooled plasma. If the aPTT corrects, a factor deficiency is present. If the aPTT remains prolonged, an inhibitor is present. Given her lack of bleeding symptoms, a lupus anticoagulant (which paradoxically causes thrombosis in vivo but prolongs the aPTT in vitro) or Factor XII deficiency (which does not cause clinical bleeding) are the most probable underlying etiologies. The mixing study is the mandatory algorithmic gateway to determine which specific subsequent assays to order.

Step-by-Step Solution

1
Analyze the patient's clinical presentation and bleeding history.
The patient is completely asymptomatic with a negative bleeding history, having tolerated significant hemostatic challenges (deliveries, dental extractions) without issue.
A negative clinical bleeding history helps rule out severe congenital bleeding diatheses and points toward an in-vitro laboratory phenomenon or a non-hemorrhagic condition.
2
Interpret the preoperative laboratory results.
The patient has an isolated, significantly prolonged aPTT. PT, platelet count, and hemoglobin are completely normal.
An isolated prolonged aPTT indicates an issue specifically within the intrinsic or common pathways of the coagulation cascade.
3
Identify the standard diagnostic algorithm for an isolated prolonged aPTT.
The established initial step for evaluating an unexplained prolonged aPTT is a 1:1 mixing study with normal pooled plasma.
The mixing study broadly categorizes the etiology. If the aPTT corrects, it indicates a factor deficiency (the normal plasma supplies the missing factors). If it fails to correct, it indicates the presence of a circulating inhibitor (which inhibits the factors in the normal plasma as well).

Key Concept

The initial laboratory evaluation of an isolated prolonged aPTT requires a 1:1 mixing study to differentiate between coagulation factor deficiencies and circulating inhibitors before ordering specific confirmatory assays.
Rate this question

Topics

Ordering Laboratory Tests

More questions from this topic