Variability of anti-PF4/heparin antibody results obtained by the rapid testing system ID-H/PF4-PaGIA.
BACKGROUND
Recent studies have shown that a low clinical pretest probability may be adequate for excluding heparin-induced thrombocytopenia. However, for patients with intermediate or high pretest probability, laboratory testing is essential for confirming or refuting the diagnosis. Rapid assessment of anti-PF4/heparin-antibodies may assist clinical decision-making.


OBJECTIVE
To evaluate the performance of rapid ID-H/PF4-PaGIA. In particular, we verified reproducibility of results between plasma and serum specimens, between fresh and frozen samples, and between different ID-H/PF4-polymer lots (polystyrene beads coated with heparin/PF4-complexes).


METHODS
The samples studied were 1376 plasma and 914 corresponding serum samples from patients investigated for suspected heparin-induced thrombocytopenia between January 2000 and October 2008. Anti-PF4/heparin-antibodies were assessed by ID-H/PF4-PaGIA, commercially available ELISAs and heparin-induced platelet aggregation test.


RESULTS
Among 914 paired plasma/serum samples we noted discordant results (negative vs. low-titre positive) in nine instances (1%; 95%CI, 0.4-1.6%). Overall, agreement between titres assessed in plasma vs. serum was highly significant (Spearman correlation coefficient, 0.975; P < 0.0001). Forty-seven samples tested both fresh and after freezing/thawing showed a good agreement, with one discordant positive/negative result (Spearman correlation coefficient, 0.970; P < 0.0001). Among 1376 plasma samples we noted a strikingly variable incidence of false negative results (none - 82%; 95%CI, 66-98%), depending on the employed ID-H/PF4-polymer lot. Faulty lots can be recognized by titrating commercial positive controls and stored samples of HIT-patients.


CONCLUSIONS
Laboratories performing the assay should implement stringent internal quality controls in order to recognize potentially faulty ID-H/PF4-polymer lots, thus avoiding false negative results.
Schneiter
S
S
Department of Haematology and Central Haematology Laboratory, Inselspital, Bern University Hospital and University of Bern, CH - 3010 Bern, Switzerland.
Colucci
G
G
Sulzer
I
I
Barizzi
G
G
Lämmle
B
B
Alberio
L
L
D000937:Antigen-Antibody Reactions; D016477:Artifacts; D001323:Autoantibodies / Q000097:blood*; D002498:Centrifugation; D042241:Early Diagnosis; D004797:Enzyme-Linked Immunosorbent Assay; D005615:Freezing; D005782:Gels; D006493:Heparin / Q000276:immunology* / Q000494:pharmacology; D006801:Humans; D007163:Immunosorbent Techniques / Q000295:instrumentation; D008863:Microspheres; D010974:Platelet Aggregation / Q000187:drug effects; D010978:Platelet Factor 4 / Q000276:immunology*; D010979:Platelet Function Tests; D011137:Polystyrenes; D016553:Purpura, Thrombocytopenic, Idiopathic / Q000097:blood* / Q000175:diagnosis; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013048:Specimen Handling
D001323:Autoantibodies; D005782:Gels; D011137:Polystyrenes; D010978:Platelet Factor 4; D006493:Heparin