Abstract 1448

BACKGROUND:

The new IgG specific ELISA for anti-platelet factor 4 (PF4)/heparin antibody is recently introduced for the diagnosis of heparin-induced thrombocytopenia (HIT). It is as sensitive but more specific than the poly-specific IgG, M, A ELISA. About 50% of patients testing positive for the anti-PF4/heparin antibody have thrombocytopenia alone as initial manifestation of HIT without thrombosis (isolated HIT). These patients are thought to be at high risk (≂f30%) for developing thrombosis within next 28 days. The incidence of isolated HIT using IgG-specific ELISA is unknown.

OBJECTIVES:

Evaluate the incidence of isolated HIT using the IgG specific anti-PF4/heparin ELISA and the risk of developing subsequent thrombosis in these patients.

PATIENTS AND METHODS:

We performed a retrospective observational study of patients being tested for the anti-PF4/heparin antibody at our institution from December 2008 to May 2010 using the IgG-specific anti-PF4/heparin ELISA with a two-step assay using high dose heparin in the second step to demonstrate heparin specificity. A positive test was defined as optical density (OD) >0.4 with >50% inhibition.

RESULTS:

319 patients were tested during the study period. 23 (7.2%) patients were diagnosed with HIT based on clinical findings (4T score) and ELISA. 16/23 (70%) had thrombosis at diagnosis whereas 7/ 23 (30%) had isolated HIT. All 7 patients with isolated HIT had follow up data for at least 3 months after diagnosis. Only 2/7 were treated with direct thrombin inhibitors (DTI) and 4/7 were treated with warfarin for at least 1 month. While only 3 of the 7 isolated HIT patients had compression ultrasonography to rule out occult lower extremity thrombus, none of these 7 patients (including 4 with an OD of >1.0) developed symptoms or signs of thrombosis in the subsequent 3 months after the diagnosis of isolated HIT.

CONCLUSION:

The incidence of isolate HIT in this study (7.2%) is significantly lower than previously reported by others and our own historical patients (57.4%, Altuntus et al, Euro J of Haematology 2008) using the IgG, M, A poly -specific anti-PF4/heparin ELISA (z value 2.092, p=0.036). It is possible that many patients previously thought to have HIT by the poly-specific anti-PF4/heparin ELISA assay were false positive. Using the IgG-specific anti-PF4/heparin ELISA not only improved the specificity of the ELISA assay, it also reduced the number of patients with isolated HIT significantly who also seem to have lower risk of developing subsequent thrombosis.

Pt4T scoreOD% inhibitionNadir plateletDTIWarfarin after DTIThrombosis at follow up
1 5 1.877 97 52 Yes Yes No 
2 6 2.098 92 40 Yes Yes No 
3 4 0.511 97 79 No No No 
4 4 0.479 105 107 No No No 
5 4 1.227 105 57 No No No 
6 4 1.213 100 104 No Yes No 
7 4 0.492 84 50 No Yes No 
Pt4T scoreOD% inhibitionNadir plateletDTIWarfarin after DTIThrombosis at follow up
1 5 1.877 97 52 Yes Yes No 
2 6 2.098 92 40 Yes Yes No 
3 4 0.511 97 79 No No No 
4 4 0.479 105 107 No No No 
5 4 1.227 105 57 No No No 
6 4 1.213 100 104 No Yes No 
7 4 0.492 84 50 No Yes No 
View Large
Disclosures:

No relevant conflicts of interest to declare.

Topics:
enzyme-linked immunosorbent assay, heparin, immunoglobulin g, thrombocytopenia, heparin-induced, thrombosis, antibodies, follow-up, warfarin, direct thrombin inhibitor, false-positive results

Author notes

*

Asterisk with author names denotes non-ASH members.

© 2010 by The American Society of Hematology
2010
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