Indian Journal of Pathology and Microbiology

: 2014  |  Volume : 57  |  Issue : 1  |  Page : 31--38

Heparin: Induced thrombocytopenia: Incidence and laboratory approach to diagnosis in Indians

Jyoti Kotwal1, Sanjeev Chaudhary2, Madakshira Gopal Manoj3, Shahbaz Hasnain4, Velu Nair5, Manoj Luthra6,  
1 Senior Advisor, Department of Pathology, Army Hospital (Research and Referral) Delhi, India
2 Department of Pathology, CRPF, Pune, Maharashtra, India
3 Graded Specialist, Department of Pathology, 160 Military Hospital, Silchar, Pune, Maharashtra, India
4 Senior Advisor, Department of Anaesthesia, Military Hospital (Cardiothoraic Centre), Pune, Maharashtra, India
5 Dean and Consultant, Department of Internal Medicine, Armed Forces Medical College, Pune, Maharashtra, India
6 Consultant, Department of Cardio-Thoraic Surgery, Armed Forces Medical College, Pune Maharashtra, India

Correspondence Address:
Jyoti Kotwal
Senior Advisor (Pathology and Hematopathology), Department of Pathology, Army Hospital (Research and Referral), New Delhi


Background and objectives: One of the most common complications of heparin administration is heparin-induced thrombocytopenia (HIT) which can also lead to catastrophic thrombotic events. The problem of identifying the cause of thrombocytopenia, as due to heparin, in patients with multiple co-morbid conditions is very essential for management. Thus, the laboratory investigations for diagnosis of HIT play a pivotal role. The objective of the study was to arrive at the incidence of HIT in ethnic Indian population and provide a decision after analysis of tests used to diagnose HIT. Materials and Methods: 125 consecutive patients (Power of study being 80%) undergoing open heart surgery and receiving unfractionated heparin were taken as subjects. Blood samples were collected a day before the surgery and days 1, 3, 5 and 7 after surgery. The cases were categorized into probable and unlikely groups depending on the clinical presentation and degree fall of platelet count. Anti-heparin PF4-associated antibodies were detected using rapid-ID gel microtyping system and ELISA tests. HIT was also tested using functional assays:- heparin-induced platelet aggregation test (PAT) and the rapid luminographic assay of heparin-induced ATP release. Results: Of the 125 patients, 11 patients were clinically labeled as probable HIT and 29 patients were clinically labeled as unlikely HIT. There were seven confirmed cases of HIT cases that were positive for one functional and one immunological assay. Only one case of HITT was encountered. Accordingly, the incidence of HIT was found to be 5.6 % and that of HITT to be 0.8%. ELISA tests were positive in 21 cases (17%) which demonstrated the presence of anti-HPF4 antibodies in non-HIT cases as well. It was found that the rapid gel test had sensitivity comparable to functional assay with better specificity than ELISA. Interpretation and conclusions: Incidence of HIT in ethnic Indian population is 5.6%. Patients with a drop of >50% in platelet count should be perused as a likely candidate of HIT. These cases should be subjected to the ID-HPF4 antibody assay as this is a rapid test, can be done for individual cases, and has better specificity and similar sensitivity than ELSIA. Cases with clinically probable HIT and a positive ID-HPF4 assay can be taken as confirmed cases of HIT. However, cases clinically unlikely for HIT and a positive ID-HPF4 assay should be subjected to another test to establish the diagnosis of HIT.

How to cite this article:
Kotwal J, Chaudhary S, Manoj MG, Hasnain S, Nair V, Luthra M. Heparin: Induced thrombocytopenia: Incidence and laboratory approach to diagnosis in Indians.Indian J Pathol Microbiol 2014;57:31-38

How to cite this URL:
Kotwal J, Chaudhary S, Manoj MG, Hasnain S, Nair V, Luthra M. Heparin: Induced thrombocytopenia: Incidence and laboratory approach to diagnosis in Indians. Indian J Pathol Microbiol [serial online] 2014 [cited 2021 Apr 23 ];57:31-38
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Full Text


Heparin is one of the most commonly used anticoagulants administered to patients during cardiac catheterization, angioplasty, cardiopulmonary bypass and the treatment of unstable angina and myocardial infarction. The clinical advantages of heparin include its immediate onset after intravenous administration, relative ease of titration and rapid reversibility with protamine. Indeed, in some situations, such as cardiopulmonary bypass, no substitute for heparin is available for general use. [1],[2] However, one of the most common side effects is heparin-induced thrombocytopenia (HIT). HIT occurs in 5% to 10% of the patients receiving unfractionated (UF) heparin, typically within 4-15 days after starting treatment, but as early as 1 day in those who have prior heparin exposure. [3] The disorder is associated with development of multiple and recurrent arterial and venous thromboembolic events - heparin-induced thrombocytopenia-thrombosis (HITT) and a mortality rate up-to 30%. [3],[4] Many of the patients receiving heparin have co-morbid conditions that pre-dispose them to thrombocytopenia resulting from other causes i.e., sepsis, DIC, pharmacologic agents, effect of extracorporeal circulation in open heart surgery, and it may be difficult to be certain of the diagnosis of HIT. In these circumstances, it is necessary to stop heparin therapy as quickly as possible if HIT is present and thus laboratory diagnosis of the condition becomes important. [3] The gold-standard test is a serotonin release assay, but this test requires a specialized laboratory for the radioactivity assay. Literature has found that the platelet aggregation test is comparable to the serotonin release assay. [5]

Western literature has reported the incidence of HIT to be 5-10% though the antigenic positivity has been reported in 40-50% cases of coronary bypass patients getting fractionated heparin. [2] Only one work on HIT from India has been carried out in a sample of 33 patients getting heparin where the incidence of HAT and HIT was reported as 30% and 15%, respectively. [6] Is the incidence of HIT so high in India? And what would be the cost and time-effective method for laboratory diagnosis formed the research question for this study. In view of the above, a structured study with adequate sample size was designed and undertaken to provide data about HIT in Indian ethnic population and decision analysis of the tests to be used for diagnosis of HIT in patients undergoing open heart surgery.

The aim and objectives of our study were to study the incidence of heparin-associated antibodies resulting in HITT as a result of UF heparin. We also endeavoured to compare the efficacy of the two antigenic assays, i.e. rapid ID gel Microtyping system and ELISA test which pick up anti-heparin PF4 (anti-H PF4)-associated antibodies with the functional assays - heparin-induced platelet aggregation test (PAT) and the new rapid luminographic assay of heparin-induced ATP release in the diagnosis of HIT.

 Materials and Methods

As the primary objective was incidence of HIT in Indians the sample size was calculated as 122 as that would have enough power (80%) to give valid estimates at 95% confidence level about occurrence of HIT. Thus, 125 consecutive patients undergoing open heart surgery and getting fractionated heparin were studied. The case details were recorded. Blood samples from patients undergoing open heart surgery receiving heparin were collected in EDTA, sterile (tube without anticoagulant) and 3.8% sodium citrate vacutainers. Samples were drawn with a minimum of trauma or stasis at the venipuncture site. Blood samples were collected prior to surgery and on day 1, 3, 5 and 7 after surgery in EDTA vacutainers for platelet count done on Sysmex KX 21 automated haematology counter and subsequently confirmed on peripheral blood smear. HIT was suspected in cases with ≥50% drop in platelet count from base line levels.

Blood samples were also collected in 3.8% sodium citrate in 9:1 ratio and centrifuged for 15 minutes at 2500g; supernatant plasma was collected into a polypropylene plastic tube. The tubes were properly labeled including patient's name and sample type, heat inactivated for 60 minutes at 56°C. Samples were stored at -20°C for the PF4-heparin antibody test by the enzyme immunoassay. For the PAT, platelet-rich plasma (PRP) were made from a blood sample from healthy donors who were not receiving any medications 2 weeks prior to the test, not consuming alcohol and not smoking. Heat inactivated plasma was stored at -70°C for ELISA for anti-HPF4 antibodies, PAT and luminographic test for ATP release. The ID gel test was carried out on fresh sera on the same day. Blood samples were collected in 3.8% sodium citrate in a 9:1 ratio and centrifuged for 15 minutes at 100g to make PRP. Supernatant plasma was removed into polypropylene plastic tubes. The tubes were properly labeled including name and sample type. The remaining blood was centrifuged at a 2500g for 15 minutes to obtain platelet-poor plasma (PPP). If the platelet count on the PRP was less than 350 10 9 /L, another donor was obtained. If the platelet count was more than 500 10 9 /L, PPP was added in a volume calculated to bring the final platelet count to approximately 350 10 9 /L. Functional ability of these platelets were tested by any normal agonist.

Dilutions of UF heparin (25 μl) were added to a mixture of donor PRP (135 μl) and patient's stored plasma (90 μl), to test at least two final concentrations, 0.1 and 1 IU/ml. All reactions were performed in a Chrono-Log model series 500 aggregometer with stirring at 37°C and aggregation was continuously monitored for at least 10 minutes after addition of heparin to each cuvette. The aggregometer was previously calibrated to 100% light transmission with PPP of a donor. Platelet aggregation with an increase in light transmission of at least 20% and with a sharp slope was considered as positive. The aggregation of normal PRP with patient PPP in a saline solution served as a control for heparin-independent aggregating factors.

Luminographic detection of HIT antibody-induced platelet activation was also done. The reaction mixture consisted of 200 μl PRP, 50ul test sera, 5ul low-dose (0.5 U/ml) or high dose (100 U/ml) porcine heparin and 15 μl luciferin/luciferase reagent (Chronolog Corporation). Experiments were performed at 37°C using a Chrono-Log model Series 500 lumi-aggregometer linked to a model Aggro/link data reduction system. Experiments were performed with 20 minute pre-incubation of the reactants at room temperature prior to stirring (1000 rpm) and subsequent addition of luciferin/luciferase. Monitoring of the aggregation and ATP release response was done over 15 minutes. In an attempt to identify positive from negative HIT results, the ATP release in the presence of low-dose (0.5 U/ml) heparin was expressed as a ratio over ATP release in the presence of high dose (100 U/ml) heparin. A sample was considered positive if the ratio of low-dose to high-dose heparin antibody-induced ATP release was >5. The amount of heparin-dependent antibody-induced ATP release was quantitated by comparison of the light flash intensity to that produced with known concentrations of ATP.

The detection of antibodies to heparin/PF4 complex was performed by Diamed ID Micro-gel typing system and enzyme immunoassay (ELISA) by Asserachrom HPIA kit test.


The present study involved analysis of 125 patients who had undergone open heart surgery and received intraoperative heparin in an urban tertiary care center between November 2007 and December 2008. The age of the patients ranged from 20 years to 77 years [Table 1].{Table 1}

Of these 125 patients, 99 (79%) were males. There were 83/125 patients (66%) with coronary artery disease (CAD) who had undergone coronary artery bypass grafting (CABG) and 42 (34%) patients with RHD with/or valvular disorder who had undergone valve replacement (MVR, AVR or DVR) surgery [Table 2].{Table 2}

Among the co-morbidities, 18 patients were suffering from diabetes mellitus, 26 cases were hypertensive, 11 patients were having both diabetes and hypertension and 70 patients were old cases of pulmonary Koch's with bronchial asthma [Table 3].{Table 3}

When these 125 patients were investigated for thrombocytopenias, 11 (9%) developed thrombocytopenia post-surgery (the criteria for thrombocytopenia was taken as ΃50% drop in platelet count from the base line value). These 11 patients were clinically labeled as probable HIT which is defined as thrombocytopenia in patients while receiving heparin and other causes of thrombocytopenia excluded. Twenty-nine patients developed thrombocytopenia with drop in platelet count >20% but <50% from the baseline value and these patients were clinically labeled as unlikely HIT as some amount of thrombocytopenia occurs in post CABG cases due to extracorporeal circulation during surgery . In rest of the 85 patients there was no drop in platelet count and platelet count in these patients were within normal range [Figure 1].{Figure 1}

Out of 11 patients with clinically probable HIT [Table 5], 8 patients became thrombocytopenic on 5-7th post-operative day, 2 patients on 3rd post-operative day and 1 patient on 1st post-operative day [Table 4].{Table 4}{Table 5}

In patients defined as unlikely HIT, thrombocytopenia occurred in 1st-3rd post-operative day in most of the cases. Out of 11 patients with probable HIT, 5 patients (45%) were in the age group of 61-80 years [Figure 1]. 9/11 patients were males.

In these 11 patients, the average baseline platelet count observed was 189000/mm 3 (range 130000 to 252000/mm 3 ) and average lowest platelet count observed was 85500/mm 3 (range 60000 to 120000/mm 3 ). Two patients expired, one as a result of sepsis and the other due to pulmonary thromboembolism.

Samples of all patients were assessed with the heparin-induced platelet aggregation test (PAT), HPF4 ELISA and ID-HPF4 antibody test for detection of heparin-associated thrombocytopenia. Further we employed a method of detection based on lumi-aggregometry (i.e. luminographic detection of platelet-dense granule ATP release as an end point of HIT antibody mediated platelet activation) and results are compared with the PAT. The functional assays and the ELISA tests were run in parallel. For the ID-HPF4 antibody test, sera were investigated separately. The ID-HPF4 antibody tests were performed on the very day of collection of patient's sample as single test can be run.

Comparison of PAT results with luminographic results

Of 11 sera from patients with probable HIT (drop in platelet count >50% from base line value), 7 sera were positive for HIT antibodies in the PAT. All sera except one tested positive by the PAT correspond to a positive luminographic result. Of the remaining samples, 4/4 were found to be negative by both PAT and luminography with the remaining one which was tested positive by the PAT found to be negative by luminography. In patients group with unlikely HIT and patients not having thrombocytopenia, in both the functional assays, all 118 sera were HIT antibody negative [Table 6] and [Table 7].{Table 6}{Table 7}

Comparison of PAT results with HPF4 ELISA

and ID-HPF4 antibody test

Of 11 sera from patients with probable HIT, 7 sera were positive for HIT antibodies in the PAT and 4 sera were antibody negative in the PAT. In contrast, the HPF4 ELISA showed 10/11 positive sera (91% positive results). In the ID-HPF4 antibody test, 8/11 sera were detected as HIT antibody positive, and 3 sera showed negative results [Figure 1].

In patients group with unlikely HIT, in functional assay, all 29 sera were HIT antibody negative. In contrast, in the HPF4 ELISA, 5/29 sera showed positive results (17%). In the ID-HPF4 antibody test, 27/29 sera showed negative results, only one serum each was positive and indeterminable [Figure 1].

In the group of 85 patients not having HIT, all sera remained HIT antibody negative in PAT and also showed negative results in the ID-HPF4 antibody test. Using the HPF4 ELISA, 4/85 sera (5%) gave a positive result [Figure 1]

Taking the presence of positivity of one functional test and one immunological assay result in these patients to be confirmatory of HIT, we confirmed 7 cases of HIT in 125 patients. Two of the patients with confirmed HIT (case no 42 and 55) succumbed on 3rd post-operative day, one due to development of sepsis, bleeding, leading to persistent hypotension, hypoperfusion and shock and the other due to acute lung syndrome, pulmonary thromboembolism, arterial thrombosis and DIC. Thus, HITT developed in one of our patients with confirmed HIT who succumbed due to this complication. We calculated the sensitivity and specificity of the four assays performed along with positive and negative predictive values [Table 8].{Table 8}

Comparison of HPF4 ELISA and ID-HPF4 showed that their diagnostic testing parameters (mainly specificity and positive predictive value) were different and these were statistically significant (chi square, Yates corrected = 62.31, P=0.001). As observed in the study the PAT is positive in all confirmed HIT cases and negative in all non-HIT cases thus making it in the present study to be a very highly specific and sensitive test for detection of HIT. The reason for getting this high sensitivity is due to meticulously done PAT in two final concentrations of 0.1 and 1.0 IU/ml. The luminographic test was negative in one of seven patients with confirmed HIT, thus sensitivity was 86%. The negative predictive value was 99% which was comparable to that of PAT (100%) in our study. On the other hand, no positive results were noted in non-HIT cases; thus both specificity and positive predictive value were 100%.

The immunologic assay of ID-HPF4 antibody test was positive in 6 of 7 confirmed HIT cases, thus sensitivity was 86%, On the other hand, 4 positive results were noted in 114 non-HIT cases, thus specificity and positive predictive value were 97% and 60% respectively. The other immunologic assay of HPF4 ELISA (Asserachrom) was positive in 6 of 7 cases of confirmed HIT (sensitivity 86%). In 15 of 118 of non-HIT cases significant levels of antibodies to HPF4 were detected; thus, the specificity of the ELISA was 87%. Under this condition, negative and positive predictive values were 99% and 28% respectively.

As observed, the overall agreement between the PAT and HPF4 ELISA in 11 patients with probable HIT was 64% which was higher between PAT and ID-HPF4 gel immunoassay (73%). We therefore postulate that a combination of two tests might improve biologic diagnosis of HIT.

[Figure 2] and [Figure 3] show test results by ID-HPF4 card and PAT in two our patients with probable HIT.{Figure 2}{Figure 3}


The diagnosis of HIT is based on clinical criteria, but in many patients the cause of thrombocytopenia is not heparin. To discard all hypotheses as a cause other than HIT, physicians need highly sensitive tests. On the other hand, procedures with high degree of specificity are necessary to confirm the diagnosis.

The present study was carried out in 125 consecutive patients on heparin which is an adequate sample size with power of 80% in 95% confidence level to give statistically valid results on incidence of HIT in patients receiving UF heparin and comparison of various tests for diagnosis of HIT/HITT. On follow up, out of 125 patients 11 developed thrombocytopenia with >50% decrease in platelet count from base line value and were diagnosed as probable HIT because of this. Thrombocytopenia with >30% fall but <50% fall from the baseline value was seen in 29/125 patients (23%) getting heparin. Thus, probable HIT was seen in 11/125 patients (9%). In these 11 patients the average baseline platelet count observed was 189000/cumm (range 130000 to 252000/cumm). In all patients, a progressive or rapid thrombocytopenia (average lowest platelet count: 85500/cumm) occurred between 5 and 7 days in 8/11 cases (83%) after beginning of the subcutaneous heparin protocol. However, on the basis of clinical findings and presence of at least one immunological test with one functional assay for HIT being positive, seven patients were diagnosed as confirmed HIT. Thus, the incidence of HIT in the population studied is 5.6% (7/125). The incidence seen in our study is comparable to the incidence of HIT reported in world literature. [2],[3],[7],[8]

However, the complication of HITT was seen only in one case (case no 55) who developed acute lung syndrome, had pulmonary thromboembolism and expired as a consequence of the disease. Thus, the incidence of HITT in our study is 0.8% (1/125).

The incidence of HITT in cardiopulmonary bypass has been reported as 0.097% by Kalangos et al., [9] who found 5 cases of HITT in a series of 5120 cases receiving heparin. Our result is comparable to those of Kapsch et al, [10] who reported the incidence rate of 0.6%. However, as observed by us and earlier workers, [1],[7] the complications of HITT are likely to be fatal unless treated immediately by discontinuation of heparin therapy and institution of alternate anticoagulation.

In our diagnosed cases of HIT (n=7), we had only one HITT which was fatal. In this case, the thrombocytopenia occurred on third day with evidence of pulmonary microemboli with D-dimer test being positive. Both the functional and immunological assays for HIT were positive in this case. The probable cause of early catastrophic HITT was earlier exposure to heparin used for intravenous flushing. Probably this patient was already immunized and exposure to large dose of soluble heparin might have led to catastrophic increase in H/PF4 antibodies leading to thrombocytopenia with thrombotic complications.

The other case which was fatal was a 64 yrs old male who developed severe thrombocytopenia on third post-op day had features of sepsis, bleeding with persistent hypotension and succumbed. Though all four assays were positive, the patient did not develop any arterial or venous thrombosis. D-dimer test was negative. Thus, this patient was a case of HIT who succumbed to sepsis, bleeding and surgery related complications and did not develop HITT. In these cases, it is possible that endothelial or endocardial injury may predispose patients with HIT to thrombosis. [11] Cines and Tannenbaum [12] found that a heparin-dependent platelet antibody crossreacts with endothelial cells, which may serve to sensitize susceptible patients to heparin.

The incidence of HIT has been reported to be quite high by Kannan et al, [13] where they studied 33 cases on heparin. In this study, they found thrombocytopenia in 30% of cases and diagnosed HIT in 15% (5/30) cases as compared to 5.6% (7/125) in our study. Considering the fact that, it was a pilot study with a small sample size, it may have given higher incidence. The results by us of 5.6% incidence rate are realistic and similar to the other studies as already quoted. Power of our study is 80% in 95% confidence level as our sample size is 125.

Although a "gold standard" diagnostic test is considered to be the serotonin release assay (SRA), [5] we compared the efficacies of more commonly employed functional tests, i.e. heparin-induced platelet aggregation test (PAT) and luminographic ATP release assay with the immunologic/antigenic assays, i.e. the ELISA-HPF4 and newly employed ID-HPF4 antibody test for detection of heparin-induced antibodies. Combination of one functional assay with one antigenic assay in a clinical setting of probable HIT was taken as confirmation of HIT and was considered a gold standard in our study. Our study has shown the PAT test which can be readily performed in laboratory with an aggregometer to be the most sensitive (sensitivity 100%) and specific (specificity100%) test equivalent to the gold standard. The luminographic test on Chrono-log series 500 whole blood lumi-aggregometer was not as sensitive (sensitivity 87%) as PAT but was highly specific (specificity 100%) and can be recommended as a confirmatory test. Our findings on luminographic test are comparable to that of a study conducted by Stewart et al, [14] in which they found the test to be comparable to the gold-standard SRA. As for the PAT test, earlier studies found sensitivity of 39% to 81%, [15] 91% [8] and specificity of 82% to 100%, [8],[15],[16] but we got a sensitivity and specificity of 100% in the meticulously done PAT test. We had selected a panel of reactive donors for PRP from a laboratory staff to carry out the test.

Considering the fact that we had 11 cases of probable HIT, the test may be carried out in larger samples with clinically probable HIT to see reproducibility of sensitivity and specificity of PAT. However, our sample size of 125 consecutive patients is an adequate sample size with a power of 80% in 95% confidence level.

We have also analyzed the sensitivity and specificity of the two immunological tests. HPF4 ELISA (Asserachrom, Stago) has been used in a number of studies. [8],[17],[18],[19] Our study has found that this had a good sensitivity of 86%, but it was not so specific (87%). Compared to this the newly introduced ID-HPF4 antibody test system using gel card had a similar sensitivity (86%), but much higher specificity (96.6%) as it gave less false positive results. The positive predictive value of HPF4 ELISA was very low (28%) as compared to that of the ID-HPF4 antibody test (60%). The difference in positive predictive value between the two was highly significant (P=0.001). Thus, suggesting that the ID particle gel immunoassay is a better test as compared to the HPF4 ELISA.

The other advantages of the ID-HPF4 antibody test are that it can be done within 20 minutes on single samples, enabling to provide rapid results for clinical decision making. The ELISA test takes much longer time to perform and analyze the results (at least 2.5 hours) and the plates have six strips of eight wells in non-releasable pouches. So one has to have adequate numbers of samples to carry out the tests and the results cannot be readily available for clinical decisions. The better specificity of ID particle gel immunoassay is due to use of only IgG detection unlike Asserachrom ELISA which also allows detection of low-titer naturally occurring antibodies, like immunoglobulin classes M and A and thus give false positive results. Usually antibodies of the IgG class bind to multi-molecular complexes of platelet factor 4 and heparin and this is the recognized cause of HIT.

The absence of positivity of the luminographic ATP release assay in one of seven cases of confirmed HIT cases was another issue observed. However, all cases were picked up by the PAT. The literature of method of detection based on the luminographic test, Stewart et al[14] have found it to be comparable to SRA. However, no other studies could be found with use of the luminographic test. We have found it to be statistically equivalent to PAT, but there are disadvantages with this test as well as PAT that the tests are cumbersome and it is only possible to carry out the test in laboratories with aggregometers. Also, one has to have selected donors whose platelets are reactive to HIT positive sera or plasma and test should be well standardized and inter- and intra-assay coefficient of variations assessed. This is possible only in specialized laboratories. PAT can be carried out in normal aggregometers but luminographic ATP release requires specialized Lumi-aggregometer by Chrono-log Corporation with a luminescence mode. This is not readily available in most of the laboratories thus limiting the use of this test.

More efficient methods of laboratory diagnosis of HIT continue to be perused. As more is learned about the optimization of the tests, such as polymorphism of the FcgRII influencing donor platelets, the sensitivities will continue to improve. Because no commonly used test is absolute, clinical suspicion and judgment remains invaluable.


The incidence rate of HIT is 5.6% and HITT is 0.8% in Indians. As the thrombocytopenia can develop due to other causes, a patient should be clinically put into the category of definite/probable, possible and unlikely HIT. The ID-gel immunoassay for detection of anti-HPF4 antibodies (IgG) is a rapid test which should be immediately done in all cases of suspected HIT. In cases of probable HIT it confirms the diagnosis, but in other categories it should still be combined with a functional assay to confirm HIT.


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