title: Rapid exclusion or confirmation of heparin-induced thrombocytopenia: a single-center experience with 1,291 patients.
abstract: BACKGROUND
  The current gold-standard for diagnosing heparin-induced thrombocytopenia is the detection of platelet-activating antibodies by means of functional assays which, since they are time consuming and not widely available, are not suited to guiding acute treatment decisions. The objective of our study was to assess the ability of more rapid immunoassays to predict the presence of functionally relevant anti-platelet factor 4/heparin-antibodies.
  
  
  METHODS
  We analyzed 1,291 of 1,383 (93.4%) patients consecutively evaluated for suspected heparin-induced thrombocytopenia at our institution. Clinical pre-test probability was defined by the 4T-score. Anti-platelet factor 4/heparin-antibodies were measured with three immunoassays (ID-H/PF4-PaGIA, Asserachrom-HPIA, and GTI-PF4) and their functional relevance was assessed by a two-point heparin-induced platelet aggregation test. Performance of the immunoassays was evaluated by receiver operating characteristic analysis.
  
  
  RESULTS
  Among 1,291 patients, 96 (7.4%) had a positive heparin-induced platelet aggregation-test: 7 of 859 (0.8%) with a low, 50 of 358 (14.0%) with an intermediate, and 39 of 74 (52.7%) with a high 4T-score. Receiver operating characteristics analysis indicated that best immunoassay thresholds for predicting a positive platelet aggregation test were: Titer of 4 or more (ID-H/PF4-PaGIA), optical density more than 0.943 (Asserachrom-HPIA) and more than 1.367 (GTI-PF4). A 100% negative predictive value was observed at the following thresholds: Titer of 1 or under (ID-H/PF4-PaGIA), optical density less than 0.300 (Asserachrom-HPIA) and less than 0.870 (GTI-PF4). A 100% positive predictive value was reached only by ID-H/PF4-PaGIA, at titers of 32 or over. Positive and negative likelihood ratios were calculated for results between the thresholds with 100% negative or positive predictive value.
  
  
  CONCLUSIONS
  We show that: i) negative and weak positive results of immunoassays detecting anti-platelet factor 4/heparin-antibodies exclude heparin-induced thrombocytopenia; ii) anti-platelet factor 4/heparin-antibody titers of 32 or over (ID-H/PF4-PaGIA) have a 100% positive predictive value for functionally relevant antibodies; iii) combining the clinical pre-test probability with the likelihood ratio of intermediate immunoassay results allows assessment of post-test probability for heparin-induced thrombocytopenia in individual patients.
authors.affiliation: Department of Haematology, Central Haematology Laboratory, Inselspital, Bern University Hospital and University of Bern, Switzerland.
authors.forename: Vanessa
authors.initials: V
authors.lastname: Nellen
authors.forename: Irmela
authors.initials: I
authors.lastname: Sulzer
authors.forename: Gabriela
authors.initials: G
authors.lastname: Barizzi
authors.forename: Bernhard
authors.initials: B
authors.lastname: Lämmle
authors.forename: Lorenzo
authors.initials: L
authors.lastname: Alberio
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D057134:Antibodies, Neutralizing / Q000276:immunology; D002648:Child; D002675:Child, Preschool; D005260:Female; D006493:Heparin / Q000009:adverse effects* / Q000276:immunology; D006801:Humans; D007118:Immunoassay / Q000379:methods / Q000592:standards; D007223:Infant; D008297:Male; D008875:Middle Aged; D010974:Platelet Aggregation / Q000276:immunology; D010978:Platelet Factor 4 / Q000276:immunology; D012680:Sensitivity and Specificity; D013921:Thrombocytopenia / Q000139:chemically induced* / Q000175:diagnosis* / Q000276:immunology; D055815:Young Adult
chemical_list: D057134:Antibodies, Neutralizing; D010978:Platelet Factor 4; D006493:Heparin