High incidence of anti-heparin/platelet factor 4 antibodies after cardiopulmonary bypass surgery

Fifty-one patients undergoing cardiopulmonary bypass (CPB) were studied on day 0 and day 8 for heparin-induced thrombocytopenia (HIT). The platelet aggregation test (PAT) and tests for anti-heparin-platelet factor 4 (anti-H.PF4), anti-IL8 and anti-neutrophil activating peptide 2 (anti-NAP2) antibodies (Ab) were performed by ELISA. On day 8, 27% of patients were positive for anti-H.PF4Ab. None of these results were found to influence thrombotic complications or platelet counts after CPB. Our results suggest that IgG to H.PF4 may be considered a risk factor, but that additional factors must be required for HIT to develop. We conclude that assays based on platelet activation would be more appropriate for the diagnosis of HIT after CPB.     

The thrombocytopenia syndrome caused by heparin

Heparin represents the basis for drug treatment and prevention of thrombosis. However, heparin itself may produce side effects, among others two types of heparin-induced thrombocytopenia (HIT). The more frequent type 1 occurs in about 10% of patients within the first days of the treatment and disappears spontaneously without sequelae. HIT type 2 represents a far more serious complication occurring in 0.1-1% of patients somewhat later, between days 4 and 10 of heparin therapy, and may provoke severe arterial and venous thrombosis. Apart from the differences in clinical picture, data regarding platelet count and platelet aggregation test are significant for the diagnosis. Heparin should be discontinued immediately and another anticoagulant therapy introduced. We are presenting the case of a patient with thrombosis of the left external iliac artery following surgery for ovarian cancer as a complication of HIT type 2, which was recanalized successfully and the leg was saved.     

Reduced morbidity and mortality rates of the heparin-induced thrombocytopenia syndrome

PURPOSE: We reported a 61% morbidity rate and a 23% mortality rate for the heparin-induced thrombocytopenia (HIT) syndrome in 1983. We subsequently reported in 1987 that with early recognition, immediate cessation of the administration of heparin, and platelet function inhibition, the morbidity rate could be reduced to 23% and the mortality rate to 12%. One hundred recent cases of patients with heparin-associated antiplatelet antibodies (HAAb) have been reviewed to determine whether aggressive screening, early diagnosis, and alternate management could further reduce morbidity and mortality rates. METHODS: The consecutive records of 100 patients with positive platelet aggregation tests were reviewed. Sixty-six patients were male. The patients' ages ranged from 23 days to 92 years. The patients were from vascular (28), cardiothoracic (42), and other (30) services. HIT was suspected in patients who received heparin and had falling platelet counts, platelet counts less than 100,000/mm3, or new thromboembolic or hemorrhagic events. RESULTS: Heparin was not offered to six patients with known HAAb. Twelve patients were successfully treated with antiplatelet therapy and limited reexposure to heparin, and 75 patients were successfully treated with early diagnosis and prompt cessation of heparin. Alternate forms of anticoagulation therapy were used selectively. Seven patients had 11 complications. Three of the seven patients were treated successfully with warfarin anticoagulation and aspirin (2) or with aspirin alone (1). A fourth patient was treated with thrombectomy, hematoma evacuation, and aspirin. A fifth patient underwent thrombolysis and coronary angioplasty in addition to receiving warfarin and aspirin. The sixth patient required two thrombectomies and warfarin. A seventh patient required two thrombectomies and aspirin. HIT was responsible for one of 17 deaths. CONCLUSION: A 7.4% morbidity rate and a 1.1% mortality rate have been achieved in patients with HAAb by aggressive screening, early recognition of HIT, and prompt cessation of the administration of heparin. Platelet function inhibitors and other anticoagulants, including nonreacting low molecular weight heparin, are important adjuncts in the management of the thromboembolic disorders associated with HIT.     

The influence of cytomegalovirus on the natural history of HIV infection: evidence of rapid course of HIV infection in HIV-positive patients infected with cytomegalovirus

PROBLEM: A life-threatening complication of the thrombembolism prophylaxis with heparin is heparin-induced thrombocytopenia (HIT) type II. HIT type II is based on immunological mechanisms. Even low, subcutaneously applied doses may produce HIT type II. In those patients, continued application may cause thromboembolic complications. The most important symptom of HIT type II is a decrease of platelets. METHODS: In a prospective study, we investigated the incidence of HIT type II within the period from 01.07.95 to 30.06.96 in orthopedic patients. We also evaluated the importance of the daily platelet count from the fifth postoperative day for the early diagnosis of HIT type II and a possible reduction of the thrombosis rate. The study included 307 patients after primary implantation of hip and knee endoprosthesis and after hip endoprosthesis replacement. All patients received 3 x 5000 IU/d of unfractionated heparin subcutaneously. Whenever there was a decrease of platelets of at least 50% in relation to the preoperative value or whenever thrombembolic complications occurred, serum was analyzed by the heparin-induced platelet activation test (HIPA). RESULTS: 20 patients developed HIT type II. This corresponds to an incidence of 6.5%. 10 of the HIT type II antibody positive patients (50%) developed thrombembolic complications. 3 patients (0.9%) of the group studied developed clinically symptomatic thrombembolic complications without evidence of heparin antibodies. The total risk of getting thrombembolic complications was 4.2% (13 patients). 3.3% (10 patients) of the entire group developed HIT type II antibody associated thrombembolic complications; 1 patient died. The lethality in the HIT type II antibody positive patient group amounted to 5%. The patients with HIT type II received LMW heparinoid Orgaran (AKZO-Organon, The Netherlands) or hirudin (as a clinical trial). The comparison group (retrospective study from 17.10.92 to 16.10.93) was composed of 262 patients with the same operations and equal thromboembolism prophylaxis. The platelet count was made only as part of routine diagnostic tests. 21 patients (8.0%) developed clinically symptomatic thrombembolic complications. The difference in the thrombosis rate between these two groups of patients is statistically significant. Unrecognized HIT type II is probably the reason for the high thrombembolic complication rate in the comparison group. CONCLUSIONS: The daily platelet count from the fifth postoperative day and from the first day in case of reexposure to heparin is an important measure for the early diagnosis of HIT type II.     

Comparison of PF4/heparin ELISA assay with the 14C-serotonin release assay in the diagnosis of heparin-induced thrombocytopenia

The diagnosis of heparin-induced thrombocytopenia (HIT) may be affirmed by demonstrating heparin-dependent anti-platelet antibodies using the 14C-serotonin release assay (SRA). In this study, results of the SRA was compared with the recently described platelet factor 4 (PF4)/heparin enzyme-linked immunosorbent assay (ELISA). Compared with the SRA, the sensitivity and specificity of a PF4/heparin ELISA was 87% and 92%, respectively, using an assay developed in our laboratory; and 90% and 98%, respectively, using a commercially developed kit (Diagnostica Stago, Asnieres, France). However, antibodies to PF4/heparin were also detected in up to 8% of patients whose plasma was negative by SRA, and 23% of patients receiving heparin who were not thrombocytopenic. These data indicate that results obtained with the PF4/heparin ELISA and the SRA are generally in accord in patients with a clinical diagnosis of HIT. However, discrepant results occur in approximately 20% of cases because of the greater sensitivity of ELISA and the possible involvement of other heparin-binding proteins. The fact that each assay contributes independent information in some cases must be considered in the sequence of test performance and in providing consultation to the practicing hematologist.     

Steam-sterilizable, fluorescence lifetime-based sensing film for dissolved carbon dioxide

Heparin-induced thrombocytopenia (HIT) is a common adverse effect of heparin therapy that carries a risk of serious thrombotic events. This condition is caused by platelet aggregation, which is mediated by anti-heparin/platelet factor 4 antibodies. Sera from patients with HIT in the presence of platelets, induced the expression of E-selectin, VCAM, ICAM-1 and tissue factor and the release of IL1beta, IL6, TNFalpha and PAI-1 by human umbilical vein endothelial cells (HUVECs) in vitro and initiated platelet adhesion to activated HUVECs. These effects which occurred in a time-dependent manner were significant in the first 1-2 h of incubation and reached a maximum after 6 to 9 h. The GP IIb-1IIa receptor antagonist SR121566A which has been shown to block platelet aggregation induced by a wide variety of agonists including HIT serum/heparin, reduced in a dose-dependent manner the HIT serum/heparin-induced, platelet mediated expression and release of the above mentioned proteins. The IC50 for inhibition of HIT serum/ heparin-induced platelet dependent HUVEC activation by SR121566A was approximately 10-20 nM. ADP, but not serotonin release, also appeared to be involved as apyrase and ATPgammaS blocked platelet-dependent, HIT serum/heparin-induced cell surface protein expression and cytokine release by HUVECs. Increased platelet adherence to HIT serum/heparin-activated HUVECs was inhibited by SR121566A and, to a lesser extent, by apyrase and ATPgammaS, showing that platelet activation and release was at the origin of the HIT serum/heparin-induced expression of these proteins by HUVECs. Thus, sera from patients with HIT induced the expression of adhesive and coagulation proteins and the release of cytokines by HUVECs through the activation of platelets which occurred in a GP IIb-IIIa-dependent manner, a process that could be selectively blocked by SR121566A.     

Loss of extremities caused by heparin-induced thrombocytopenia

We report the cases of two patients who last lims as a result of heparin-induced thrombocytopenia (HIT). On the basis of these cases, the incidence, pathophysiology and the diagnosis of HIT are reviewed. For the diagnosis of HIT, the platelet aggregation test and ELISA are used. For HIT prophylaxis and treatment of thromboembolic complications is recommended Orgaran. Exact dosage schedules are provided.     

Defining an antigenic epitope on platelet factor 4 associated with heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a potentially serious complication of heparin therapy. Antibodies to platelet factor 4 (PF4)/heparin complexes have been implicated in the pathogenesis of this disorder, but the antigenic epitope(s) on the protein have not been defined. To address this issue, we studied the binding of HIT antibodies to a series of recombinant proteins containing either point mutations in PF4 or chimeras containing various domains of PF4 and the related protein, neutrophil activating peptide-2 (NAP-2). Serum samples from 50 patients with a positive 14C-serotonin release assay (14C-SRA) and a clinical diagnosis of HIT and 20 normal controls were studied. HIT antibodies reacted strongly with wild-type (WT) PF4/heparin complexes, but reacted little, if at all, with NAP-2/heparin complexes (optical density [OD]405 = 2.5 and 0.2, respectively). Alanine substitutions at three of the four lysine residues implicated in heparin binding, K62, K65, and K66, had little effect on recognition by HIT antibodies (OD405 = 2.2, 2.8, and 2.0, respectively), whereas an alanine substitution at position K61 led to reduced, but still significant binding (OD405 = 1.0). Similar studies involving chimeras between PF4 and NAP-2 localized a major antigenic site to the region between the third and fourth cysteine residues for more than half of the sera tested. This site appears to involve a series of amino acids immediately after the third cysteine residue beginning with P37. Thus our studies suggest that whereas the C-terminal lysine residues of PF4 are important for heparin binding, they do not comprise a critical antigenic site for most HIT antibodies. Rather, we propose that maintaining a region near the third cysteine residue of PF4, distal from the proposed heparin-binding domain, is required to form the epitope recognized by many HIT antibodies.     

Heparin-induced thrombocytopenia: new insights into the impact of the FcgammaRIIa-R-H131 polymorphism

Heparin-induced thrombocytopenia (HIT), a severe complication of heparin treatment, can be associated with new thrombotic complications. HIT antibodies activate platelets via the platelet Fcgamma-receptor (FcgammaRIIa), which carries a functionally relevant polymorphism (FcgammaRIIa-R-H131). The effect of this polymorphism on the clinical manifestations of HIT is controversial. We determined prospectively the FcgammaRIIa-R-H131 genotypes in 389 HIT patients, in 351 patients with thrombocytopenia or thrombosis due to causes other than HIT and without detectable HIT antibodies, and in 256 healthy blood donors. For this purpose, a novel nested sequence-specific primer-polymerase chain reaction (SSP-PCR) was developed. FcgammaRIIa-R/R131 was found to be overrepresented in the HIT patients (27%) compared with the control groups (non-HIT patients [21%] and blood donors [20%]). In a subgroup of 122 well-characterized HIT patients, the genotype distribution in patients presenting with thrombocytopenia only was compared with that of patients who developed thromboembolic complications. The frequency of FcgammaRIIa-R/R131 among patients with thrombotic events was significantly elevated (37% v 17%; P = .036). Our results indicate that genotype distribution can be correlated to the clinical outcome of patients with HIT. We speculate that the reduced clearance of immune complexes in patients with the FcgammaRIIa-R/R131 allotype causes prolonged activation of endothelial cells and platelets, thus increasing the risk for thrombotic complications.     

Antibody assay for adenoviruses and mycoplasma pneumoniae by the platelet aggregation test

The practicability of the platelet aggregation test (PAT) in routine antibody assays for adenoviruses and Mycoplasma pneumoniae is demonstrated. An increase in reliability and sensitivity was achieved by employment of EDTA for washing the platelet and by addition of albumin to the test medium. Various lots of platelets yielded reproducible PAT titres which attained 1 : 156,250 in positive patient's sera. Commercially available antigens assigned for complement-fixation (CF) tests proved to be appropriate for the PAT. Checkerboard titrations revealed that the platelet reactivity of immune complexes formed was maintained in a range of up to 625-fold alterations of the antibody: antigen ratio. The fact that PAT and CF titres did not always correlate is interpreted as being due to the differences between complement-fixing and platelet reactivities of various immunoglobulin classes.     

A fatal low-molecular-weight heparin-associated thrombocytopenia after hip surgery: possible usefulness of PF4-heparin ELISA test

In 37 patients undergoing total hip replacement, a prophylactic treatment by a low-molecular-weight heparin (LMWH) was conducted for 2 weeks. They belonged to a group of 499 patients included in a multicenter clinically controlled trial comparing two LMWHs. Blood was collected 1 day before surgery (D-1) and at D+1 or D+2 and D+5 or D+6 as well as D+10 through D+14 after surgery for determinations of platelets counts and anti-Xa. Bilateral venography was performed between D+10 and D+14. A fatal heparin-associated-thrombocytopenia (HAT) occurred on D+9 in one patient and was associated with a positive platelet aggregation test. This finding was confirmed with a recent ELISA test which evidenced a high concentration of PF4-heparin dependent antibodies 72 h before the detection of thrombocytopenia. This led us to study retrospectively PF4-heparin ELISA results by testing the plasma samples of 36 other surgical patients treated under the same conditions and during the same period (four measurements per patient). Among these patients, seven had a venous thrombotic event as a treatment failure. Although some authors claimed that some post-operative thromboses may be facilitated by the presence of heparin-dependent antibodies associated with or without thrombocytopenia, no thrombocytopenia and no positive PF4-heparin ELISA test was observed in this group. Out of the 144 tests performed in these 36 patients for the detection of PF4-heparin complexes dependent antibodies, 15 results were borderline in ten patients and three results in two patients were positive. No relation was evidenced between a positive ELISA test and the occurrence of venous thrombosis. This study points out the possible usefulness of the PF4-heparin ELISA test for HAT-antibodies detection. A daily platelet count in a postoperative patient under heparin therapy, showing thrombocytopenia associated with the detection of heparin-dependent antibodies could allow an earlier and more reliable diagnosis of HAT.     

Preventing the cardiotoxic effects of anthracyclines with Cardioxane]

An immune response to heparin, which is clinically manifested by the development of thrombocytopenia with or without thrombosis, is stimulated by a complex of heparin with platelet factor 4 (PF4). The primary thrombotic events in patients with heparin-induced thrombocytopenia (HIT) are more frequently venous than arterial. The development of antibodies, however, does not always result in thrombocytopenia or in catastrophic events. The antibodies, which are of the IgG, IgM, and IgA isotypes, can be easily measured by an ELISA that contains a complex of heparin-platelet factor 4 (PF4). Initial antibody formation can be greatly reduced by limiting the exposure to unfractionated heparin or by the use of low-molecular-weight heparin. For those patients who require anticoagulation and who have antibodies to heparin-PF4, danaparoid (Orgaran), a low-molecular weight heparinoid that does not react with the antibodies, is now commercially available; argatroban, a thrombin-specific inhibitor, can also be obtained for compassionate use. The use of these agents during anticoagulation with warfarin is preferable to the simple discontinuation of heparin and intitiation of warfarin, because the latter treatment can result in ongoing thrombosis.     

On the measurement of spontaneous platelet aggregation. The platelet aggregation test III. Methods and first clinical results

A new measuring device was developed for the study of "spontaneous" aggregating activity of thrombocytes. In the photometric platelet aggregation test (PAT III) 0.6 ml of platelet-rich plasma (PRP) are rotated in a disc-shaped cuvette at 20 rpm and 37 degrees C. Changes in optical density of PRP which are induced by the formation of platelet aggregates are continuously registered using a chart recorder. PAT III was developed for the detection of enhanced platelet aggregation, indicating a risk of thrombosis and thromboembolic complications. In 146 healthy individuals a certain percentage showed slight primary aggregation (alpha1) which in some cases was followed by marked aggregation (alpha2) at a certain time (Tr) after the beginning of rotation. The percentage of individuals showing alpha2 increased with age. An increase of plasma pH in the rotating sample, which was caused by diffusion of CO2, was an important conditioning factor for aggregation. The test results depended on the platelet count in PRP. Aggregation curves were suppressed by admixture of erythrocytes and lipid turbidity. The tendency of platelets to aggregate increased within 60-90 min following blood sampling. During this period the interval to the onset of aggregation (Tr) became shorter and the maximum aggregation speed (alpha 2) increased with time. PAT III yielded reproducible results when it was carried out more than 60 min after blood drawing. In a group of 327 diabetic patients "spontaneous" aggregation occurred more frequently in all age groups as compared with the controls. Additional equipment was available for the registration of ADP-, collagen-, or epinephrine-induced aggregation similar to Born's and O'Brien's method. The device can easily be mounted on an Eppendorf photometer without further alterations.     

Thrombocytopenia due to heparin therapy. Use of danaparoid (Orgaran), 13 monocentric cases under authorization of temporary prescription (ATU)

Since September 1994, danaparoid (Orgaran), a heparinoid, has been used in our centre to treat patients with thrombocytopenia occurring during heparin therapy and who need continuing antithrombotic therapy. We carried out a retrospective study using clinical and biological data on the first 13 consecutive patients treated with danaparoid (for 1 to 18 consecutive days). The platelet count returned to normal for ten patients, but one patient died having contracted a severe sepsis and bleeding occurred in one patient with acute renal failure. In the three other cases, the diagnosis of heparin induced thrombocytopenia (HIT) was in retrospect unlikely and the death of these patients was related to severe underlying diseases which were held responsible for thrombocytopenia. We confirm that danaparoid appears to be an effective, well-tolerated substitute for heparin in HIT patients. The French regulation Temporary Authorization for Prescribing Medicines allowed the prompt use of this as yet unmarketed drug and collection of reliable and pertinent data.     

Increase in beta-galactosidase activity in a non-isothermal bioreactor utilizing immobilized cells of Kluyveromyces fragilis: fundamentals and applications

Heparin-induced thrombocytopenia is an increasingly common side effect associated with heparin usage. In the more severe manifestation of the syndrome, patients can develop thrombosis; a 10% mortality is associated with heparin induced thrombocytopenia. To date, the therapeutic options for patients with heparin-induced thrombocytopenia are limited. Glycoprotein IIb/IIIa inhibitors have been shown to block platelet aggregation induced by a wide variety of agonists. The ability of antibody and synthetic small molecule inhibitors of glycoprotein IIb/IIIa to block in vitro activation and aggregation of platelets in response to heparin-induced thrombocytopenia positive serum/heparin was examined using flow cytometry, platelet aggregometry, and luminescence aggregometry. Abciximab, YM 337, and SR 121566A were each found to inhibit platelet microparticle formation and P-selectin expression in whole blood, in response to heparin-induced thrombocytopenia positive serum/heparin. In a platelet rich plasma system, the platelet aggregation response was inhibited by all three agents. The IC50 for inhibition of heparin-induced thrombocytopenia positive serum/heparin induced platelet aggregation by SR 121566A was 18 nM, a concentration which was 4 to 8 fold lower than that observed for collagen and arachidonic acid induced aggregation. Adenosine triphosphate release from activated platelets, as measured by luminescence aggregometry, was concentration-dependently inhibited by SR 121566A. These results suggest that glycoprotein Ilb/IIIa inhibitors may be beneficial in the management of heparin-induced thrombocytopenia and warrant further investigation.     

Stenosis of the tubular neck: a possible mechanism for progressive renal failure

OBJECTIVE: To study the influence of continuous administration of heparin on platelet function in intensive care patients. DESIGN: Prospective, serial investigation. SETTING: Clinical investigation on a surgical and neurosurgical intensive care unit in a university hospital. PATIENTS: The study included 45 patients: 15 postoperative with patients sepsis (Acute Physiology and Chronic Health Evaluation II score between 15 and 25), 15 trauma patients (Injury Severity Score 15 to 25), and 15 neurosurgical patients. INTERVENTIONS: Management of the patients was carried out according to the guidelines for modern intensive care therapy. Sepsis and trauma patients received standard (unfractionated) heparin continuously [aim: an activated partial thromboplastin time (aPTT) approximately 2.0 times normal value; sepsis-heparin and trauma-heparin patients], whereas neurosurgical patients received no heparin (neurosurgical patients). MEASUREMENTS AND RESULTS: From arterial blood samples, platelet aggregation was measured by the turbidimetric method. Platelet aggregation was induced by adenosine diphosphate (ADP; 2.0 mumol/l), collagen (10 micrograms/ml), and epinephrine (25 mumol/l). Measurements were carried out on the day of diagnosis of sepsis or 12 h after hemodynamic stabilization (trauma and neurosurgery patients) (baseline) and during the next 5 days at 12.00 noon. Standard coagulation parameters [platelet count and fibrinogen and antithrombin III (AT III) plasma concentrations] were also monitored. Heparin 4-10 U/kg per h (mean dose: approximately 500 U/h) was necessary to reach an aPTT of about 2.0 times normal. Platelet count was highest in the neurosurgical patients, but it did not decrease after heparin administration to the trauma and sepsis patients. AT III and fibrinogen plasma levels were similar in the three groups of patients. In the sepsis group, platelet aggregation variables decreased significantly (e.g., epinephrine-induced maximum platelet aggregation:-45 relative % from baseline value). Platelet function recovered during the study and even exceeded baseline values (e.g., ADP-induced maximum platelet aggregation: +42.5 relative % from baseline value). Continuous heparinization did not blunt this increase of platelet aggregation variables. In the heparinized trauma patients, platelet aggregation variables remained almost stable and were no different to platelet aggregation data in the untreated neurosurgical patients. CONCLUSIONS: Continuous administration of heparin with an average dose of approximately 500 U/h did not negatively influence platelet function in the trauma patients. Recovery from reduced platelet function in the sepsis group was not affected by continuous heparinization. Thus, continuous heparinization with this dose appears to be safe with regard to platelet function in the intensive care patient.     

Antiphospholipid syndrome. Proposition for management

Antiphospholipid antibodies (antiprothrombinase and anticardiolipin) carry with them for mothers the risks of repeated fetal loss and of disorders of the blood clotting mechanism both before and after delivery. All the same screening does not have to be carried out routinely but should be reserved for patients who have already lost one fetus (intrauterine death after 12 weeks of amenorrhoea) and/or venous or arterial thrombosis. The diagnosis depends on a strict methodology and strict criteria for making a positive diagnosis. The treatment of these antibodies (with corticosteroids and intravenous immunoglobulin) or the prevention of possible thrombotic complications (using platelet antiaggregation/heparin) has to be decided taking into account the level of antibodies, previous obstetric and thrombotic history and the lupus symptomatology as shown by the patients. The overall success rate of treatment is between 53 and 81%.     

Morphological changes in paraboloid glycogen of the accessory cone of the chick retina by exposure to light (author''s transl)

We studied five patients in whom severe thrombocytopenia developed during intermittent intravenous heparin treatment of arterial and venous thrombosis. Platelet aggregation was demonstrated when heparin (0.5 U per milliliter) was incubated with the patients' citrated platelet-rich plasma or with normal platelet-rich plasma in the presence of the patients' serum. Antiplatelet antibody was not detected in the patient globulin fractions prepared from serum collected within one week after heparin withdrawal by use of the platelet factor 3 availability technic. When the studies were repeated with modifications to detect heparin-dependent antiplatelet antibodies, positive results were obtained in four of five patients. The data suggest that a casual relation, mediated by an immune mechanism, existed between heparin therapy and thrombocytopenia, and that this syndrome may occur more often than has previously.     

Rifampicin-induced immune thrombocytopenia

Rifampicin-induced thrombocytopenia is reported in three patients with pulmonary tuberculosis. All three patients gave a definite history of having had prior exposure to rifampicin. Immunological studies in all three patients showed the presence of antiplatelet antibodies, resulting in thrombocytopenia. Moreover, binding of these antibodies to the platelet membrane was more avid in the presence of rifampicin, thereby implicating the drug. The avidity of the rifampicin-dependent antibodies was demonstrated by platelet aggregation inhibition test, and estimation of the rifampicin-dependent antibody was done by studying the platelet-associated immunoglobulin [PAlgG] by ELISA which was also used to quantitate antiplatelet antibodies. Immunofluorescence test was also performed to detect antiplatelet antibodies.     

Pitfalls in HIV testing. Application and limitations of current tests

Enzyme-linked immunosorbent assay (ELISA) and Western blot assay are the most commonly used laboratory tests for HIV infection. Both detect antibodies to HIV. ELISA results are based on detection of antigen-antibody complexes by using antibodies labeled with an enzyme that produces a color change in the presence of a specific substrate. Currently licensed ELISA tests have greater than 98% sensitivity and specificity for HIV. Western blot analysis detects antibodies to specific HIV antigens and is best used as a confirmatory test. In spite of the high sensitivity and specificity of both tests, false-positive and false-negative results do occur. Physicians should be aware of specific causes of inaccurate results. In individual cases, knowledge of the patient's history and the criteria used by the laboratory performing the test is important.