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Zorluk: ZorOrdering Laboratory Tests

A 62-year-old man is evaluated in the hospital for a decreased platelet count. He was admitted 6 days ago for a massive pulmonary embolism and was initially treated with an intravenous unfractionated heparin infusion. He was transitioned to oral warfarin 2 days ago, but the heparin infusion was continued as a bridge. His platelet count on admission was 320,000/mm3320,000/\text{mm}^3. Today's complete blood count reveals a platelet count of 140,000/mm3140,000/\text{mm}^3. His hemoglobin and leukocyte counts are normal. He has no active bleeding, and physical examination shows no new petechiae, ecchymoses, or signs of recurrent deep vein thrombosis. A review of his peripheral blood smear shows normal red blood cell morphology and confirms the thrombocytopenia without platelet clumping. Which of the following is the most appropriate next step in laboratory evaluation?

  1. Enzyme-linked immunosorbent assay for platelet factor 4-heparin antibodiesCevap
  2. B
    Functional testing with the serotonin release assay
  3. C
    Measurement of serum ADAMTS13 activity
  4. D
    Anti-platelet glycoprotein IIb/IIIa antibody testing
  5. E
    Measurement of lupus anticoagulant

Cevap

Enzyme-linked immunosorbent assay for platelet factor 4-heparin antibodies
The most appropriate initial step in laboratory evaluation for suspected heparin-induced thrombocytopenia (HIT) is an enzyme-linked immunosorbent assay (ELISA) to detect antibodies against the platelet factor 4 (PF4)-heparin complex. This patient has a high clinical probability of HIT based on a >50%>50\% drop in his platelet count occurring precisely 6 days after the initiation of unfractionated heparin, with no alternative etiology identified. Because the PF4-heparin ELISA is highly sensitive and relatively rapid, it is the preferred initial diagnostic screening test. A negative test effectively rules out HIT, whereas a positive test typically requires confirmation with a more specific functional assay before the diagnosis is definitively established.

Adım Adım Çözüm

1
Assess the clinical probability of heparin-induced thrombocytopenia (HIT) using the 4T score.
The patient has a score of 6 (High Probability): >50%>50\% drop in platelets (2 points), day 6 of heparin therapy (2 points), no new thrombosis (0 points), and no other evident cause (2 points).
A formal pre-test probability assessment guides the necessity and interpretation of specific diagnostic laboratory testing.
2
Determine the appropriate sequential laboratory evaluation for intermediate to high probability HIT.
Order an immunoassay (ELISA) for platelet factor 4 (PF4)-heparin antibodies as the initial diagnostic screening test.
The ELISA is highly sensitive and widely available, allowing rapid ruling out of HIT if negative, prior to considering more complex functional assays.

Anahtar Kavram

The initial step in the laboratory evaluation of clinically suspected heparin-induced thrombocytopenia (HIT) is an immunoassay (ELISA) to detect platelet factor 4 (PF4)-heparin antibodies. Confirmatory functional assays, such as the serotonin release assay (SRA), are reserved for patients with a positive initial immunoassay.
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Ordering Laboratory Tests

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