Quantitative interpretation of optical density measurements using PF4-dependent enzyme-immunoassays.
BACKGROUND
Many laboratories test for heparin-induced thrombocytopenia (HIT) using a PF4-dependent enzyme-immunoassay (EIA). An advantage of the EIA is its simplicity; a disadvantage is that it only indirectly detects heparin-dependent, platelet-activating antibodies ('HIT antibodies').


OBJECTIVE
To determine whether the magnitude of a positive EIA result, expressed in optical density (OD) units, predicts risk of HIT antibodies, defined as a strong-positive platelet serotonin-release assay (SRA) result (>or=50% serotonin release).


METHODS
We determined the risk of a strong-positive SRA result for five categories of OD reactivity (<0.40, 0.40-<1.00, 1.00-<1.40, 1.40-<2.00, and >or=2.00 OD units) using two EIAs (commercial anti-PF4/polyanion IgG/A/M and in-house anti-PF4/heparin-IgG).


RESULTS
For patient sera investigated for HIT antibodies, a weak-positive result (0.40-<1.00 OD units) in either EIA indicated a low probability (<or=5%) of a strong-positive SRA; the risk increased to approximately 90% with an OD >or= 2.00 units. Quantifying the EIA-SRA relationship for 1553 referred patient sera, we found that for every increase of 0.50 OD units in the EIA-IgG, the risk of a strong-positive SRA result increased by OR = 6.39 [95% confidence interval (CI), 5.13, 7.95; P < 0.0001]. For every increase of 1.00 OD units in the EIA-IgG, the risk increased by OR = 40.81 (95% CI, 26.35, 63.20; P < 0.0001).


CONCLUSIONS
The probability of HIT antibodies (strong-positive SRA result) inferred by a positive PF4-dependent EIA varies considerably in relation to the magnitude of the EIA result, expressed as OD values. In our laboratory, the probability of HIT antibodies being present reached >or=50% only when the OD level was >or=1.40 units.
Warkentin
T E
TE
Department of Pathology and Molecular Medicine, Michael G DeGroote School of Medicine, McMaster University, Hamilton, ON, Canada. twarken@mcmaster.ca
Sheppard
J I
JI
Moore
J C
JC
Sigouin
C S
CS
Kelton
J G
JG
D001774:Blood Chemical Analysis / Q000379:methods* / Q000706:statistics & numerical data; D006493:Heparin / Q000009:adverse effects*; D006801:Humans; D007124:Immunoenzyme Techniques / Q000379:methods* / Q000706:statistics & numerical data; D007074:Immunoglobulin G / Q000097:blood; D015539:Platelet Activation / Q000187:drug effects / Q000276:immunology; D010978:Platelet Factor 4 / Q000032:analysis* / Q000276:immunology; D012680:Sensitivity and Specificity; D012701:Serotonin / Q000097:blood / Q000378:metabolism; D013921:Thrombocytopenia / Q000097:blood / Q000175:diagnosis* / Q000209:etiology* / Q000276:immunology
D007074:Immunoglobulin G; D012701:Serotonin; D010978:Platelet Factor 4; D006493:Heparin