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Zorluk: ZorOrdering Laboratory Tests

A 68-year-old man is evaluated on hospital day 6 for an abnormal laboratory result. He was admitted for a severe exacerbation of chronic obstructive pulmonary disease and has been receiving intravenous methylprednisolone, azithromycin, and subcutaneous unfractionated heparin for deep venous thrombosis prophylaxis. His admission platelet count was 260,000/mm³. Today's routine complete blood count reveals a platelet count of 115,000/mm³. Hemoglobin is 13.8 g/dL and leukocyte count is 9,500/mm³. Vital signs are within normal limits. Physical examination reveals no petechiae, purpura, or new extremity swelling. The physician immediately discontinues the subcutaneous heparin and initiates therapeutic anticoagulation with argatroban. Which of the following is the most appropriate next step in laboratory evaluation?

  1. Anti-platelet factor 4 (PF4)-polyanion antibody immunoassayCevap
  2. B
    Serotonin release assay
  3. C
    Prothrombin time, activated partial thromboplastin time, and D-dimer
  4. D
    Anti-glycoprotein IIb/IIIa and anti-Ib/IX antibody testing
  5. E
    Repeat complete blood count in 24 hours

Cevap

Anti-platelet factor 4 (PF4)-polyanion antibody immunoassay
The correct next step is the anti-platelet factor 4 (PF4)-polyanion antibody immunoassay. This patient has a classic presentation of Type 2 heparin-induced thrombocytopenia (HIT), characterized by a platelet count drop of >50% occurring 5 to 10 days after the initiation of heparin. His 4Ts score is high (2 points for magnitude of fall, 2 points for timing, 0 points for thrombosis, 2 points for no other apparent cause = 6 points). For patients with an intermediate or high 4Ts score, all heparin products must be stopped immediately and a non-heparin anticoagulant (such as argatroban) should be started. The initial diagnostic laboratory test is an enzyme-linked immunosorbent assay (ELISA) to detect antibodies against the PF4-heparin complex. This test is highly sensitive; a negative result effectively rules out HIT. If the ELISA is positive, a functional assay (such as the serotonin release assay) is ordered to confirm the diagnosis.

Adım Adım Çözüm

1
Analyze the patient's clinical presentation and laboratory timeline.
The patient has an isolated >50% drop in platelets (from 260,000 to 115,000) on day 6 of unfractionated heparin therapy.
Establishing the timing and magnitude of thrombocytopenia is required to calculate the 4Ts score for heparin-induced thrombocytopenia (HIT).
2
Calculate the 4Ts score to determine pretest probability.
The score is at least 4 (2 points for >50% fall, 2 points for clear day 5-10 timing), indicating an intermediate to high probability of HIT.
The 4Ts score dictates the subsequent clinical and laboratory management algorithm.
3
Determine the appropriate diagnostic test sequencing for intermediate/high probability HIT.
The first step is to order an anti-platelet factor 4 (PF4) immunoassay (ELISA) as an initial screen.
ELISA is highly sensitive and rapid. A functional assay (like the serotonin release assay) is reserved for confirmation only if the ELISA is positive.

Anahtar Kavram

In patients with an intermediate to high pretest probability of heparin-induced thrombocytopenia (HIT), the most appropriate initial diagnostic laboratory test is the anti-PF4-polyanion antibody immunoassay, followed by a confirmatory functional assay (e.g., serotonin release assay) only if the initial screen is positive.
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Ordering Laboratory Tests

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