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PF4-Dependent P-Selectin Assay Highly Accurate in Diagnosing Heparin-Induced Thrombocytopenia
A study published in Blood established the diagnostic accuracy of a PF4-dependent P-selectin expression assay ( PEA ) for heparin-induced thrombocytopenia ( HIT ), with a sensitivity similar to that of the “ gold standard ” serotonin release assay ( SRA ).
“ The PEA is the first laboratory test shown to be as accurate as the SRA ,” study co-author Anand Padmanabhan , MD , PhD , from Mayo Clinic in Rochester , Minnesota , told ASH Clinical News . Of note , “ it is technically simpler than the SRA and uses 20-fold fewer platelets and does not require radioactive reagents .”
The study conducted by Dr . Padmanabhan and colleagues enrolled 409 consecutive adult inpatients who underwent enzyme-linked immunosorbent assay ( ELISA ) testing for suspected HIT from 2016 and 2017 at Mayo Clinic and the University of Washington .
Serum samples were obtained for immunoglobulin G-specific ELISA testing and were batchtested with PEA and SRA with the same target platelets used in paired SRA-PEA runs . Samples were classified as HIT-positive , HIT-negative or HIT-indeterminate based on 4Ts scores and laboratory values , with the following manufacturerrecommended cutoffs for test positivity :
• University of Washington : optical density ( OD ) > 0.3
• Mayo Clinic : OD ≥0.4
A total of 284 patients had low 4Ts scores ( 69.4 %), while 98 and 27 had intermediate or high 4Ts scores ( 24 % and 6.6 %), respectively . Forty-nine ELISA results were positive and 360 were negative .
Using the predefined criteria , 17 patients were considered positive for HIT . People in this group had a median PEA of 88 % and a median SRA of 69 %. These rates decreased significantly in HITindeterminate patients , to 46 % and 5 %, respectively . The researchers added that platelet activation in the PEA and SRA was not observed in the majority of samples from HIT-negative patients .
According to receiver operating characteristic curve statistics , both the PEA and SRA correctly stratified patients into disease-positive and negative groups , with similar areas under the curve :
• PEA : 0.94 ( 95 % CI 0.87-1.0 )
• SRA : 0.91 ( 95 % CI 0.82-1.0 )
“ We hope that the PEA might someday permit hospitals to perform the assay ‘ in-house ’ while eliminating the need for send-out laboratory testing in patients with possible HIT . ”
In a sensitivity analysis that considered indeterminate-HIT patients as disease-positive , 26 patients were classified as having HIT , and the PEA and SRA were again similar in terms of diagnostic accuracy , with AUCs of :
• PEA : 0.88 ( 95 % CI 0.78-0.98 )
• SRA : 0.86 ( 95 % CI 0.77-0.96 )
— Anand Padmanabhan , MD , PhD
The concordance between PEA and SRA results was high for HIT-negative patients , and slightly lower for HIT-positive patients ( TABLE ). When looking at these discordant cases , 1 patient was found to have a false-negative test . Two of the 5 HIT-positive patients were re-exposed to heparin because of negative SRA results ; both experienced a decrease in platelet counts following exposure confirming “ true ” HIT .
Dr . Padmanabhan noted that some patients with results that were “ barely ” positive by ELISA testing ( e . g ., < 1 OD ) had unequivocal HIT based on testing with PEA together with clinical history and platelet responses to heparin re-exposure . “ Thus , while the strength of ELISA ODs generally correlates with platelet-activating antibodies , there are exceptions to this which can have important management implications ,” he said .
Because the PEA requires fewer resources than the SRA , Dr . Padmanabhan indicated that this method may offer an easier option to diagnose HIT . “ The currently available laboratory testing for HIT either lacks specificity or is technically difficult to perform , often requiring a several-day delay between sample collection and final confirmation or exclusion of HIT ,” he said . “ While we cannot be certain about the impact of our findings on the clinical care of patients with suspected HIT , we hope that the characteristics of the PEA might someday permit some hospitals to perform the assay ‘ in-house ’ while eliminating the need for send-out laboratory testing in patients with possible HIT .”
In terms of limitations , Dr . Padmanabhan stated that the investigators lacked a “ gold standard ” assay for which they could have absolutely confirmed or refuted an HIT diagnosis . The reliance on clinical and laboratory criteria to classify patients as HIT-positive and HIT-negative represented an additional limitation . Dr . Padmanabhan suggested this could have led to misclassification , as some patients who were initially classified as disease-negative were later considered to have HIT on follow-up investigation .
Study authors report no relevant conflicts of interest .
Reference Bannow Samuelson BT , Warad D , Jones C , et al . A prospective , blinded study of a PF4-dependent assay for HIT diagnosis . Blood . 2020 September 8 . [ Epub ahead of print ]
TABLE . Concordance Between the PEA and SRA
SRA (%) Negative Positive
Negative 373 8 PEA (%)
Positive 10 18 PEA = PF4-dependent P-selectin expression assay ; SRA = serotonin release assay
Positive Concordance
Negative Concordance
0.692 0.974
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