Clinical Laboratory Catalog > HIT-Platelet Factor 4 (PF4) – LAB3205

HIT-Platelet Factor 4 (PF4) – LAB3205

Synonyms PF4, HIT antibody, Heparin PF4 antibody, HEPARIN-PF4 AB
Patient Preparation Test is only appropriate for patients on therapeutic heparin.
Specimen Type Sodium citrate plasma
Collection Requirements Draw one blue top tube (3.2% sodium citrate). Invert the tube three to four times immediately after collection to ensure proper mixing of blood. NOTE: The blue top tube should not be the first tube drawn if multiple tubes are being collected. If the blue top tube is the only tube to be drawn, a small amount of blood should be collected in another tube and discarded before the blue top tube is drawn.
Minimum Volume 50 uL
Cause for Rejection Microbially contaminated, hemolyzed, lipemic, icteric, or heat-inactivated serum samples may give inconsistent test results and should be avoided.
Specimen Handling/Stability Centrifuge, remove serum/plasma from cells ASAP. If testing is unable to be completed within 4 hours, freeze at -20 °C
Specimen Storage Stability Samples may be frozen and maintained at -20°C for two freeze/thaw cycles.
Test Availability Testing performed daily on first shift at approximately 1pm.
Reference Range

Negative
Interpretation

Results are reported as Negative or Positive.

The ‘4T’ scoring system of Warkentin is widely used to predict the risk that an individual with thrombocytopenia may have HIT.

T Score
  2 1 0
Thrombocytopenia >50% fall in platelet count or a platelet nadir of 20-100 x 109/L 30-50% fall  in platelet count or a platelet nadir of 10-19 x 109/L <30% fall in platelet count or a platelet nadir of <10 x 109/L
Timing Onset with 5-10 days of exposure to heparin or

<1 day if previous exposure to heparin within 100 days

 Unclear-platelet count falls after 10 days Platelet count falls too early and without recent exposure to heparin
Thrombosis New thrombosis, skin necrosis Progressive or recurrent thrombosis, some skin lesions (erythema) none
Other causes of thrombocytopenia are not evident No other cause identified Possible other cause Other cause clearly identified

A score of ≤3 indicates a low pretest probability of Type II HIT (<5% chance of HIT)

A score of 4-5 indicates an intermediate risk

A score ≥6 is associated with a very high risk of HIT.

The American College of Chest Physicians Guidelines for the diagnosis of Type II HIT has identified the following diagnosis criteria:

  1. Thrombocytopenia- a fall in platelet count that occurs 4-10 days after beginning heparin therapy
  2. A fall in platelet count to <150 x 109/L or to 50% of the baseline platelet count
  3. The exclusion of other causes of thrombocytopenia
  4. The recovery of the platelet count to >90% of the baseline after heparin is stopped