Thrombocytopenia in hospitalized patients: approach to the patient with thrombotic microangiopathy

Thrombotic microangiopathy (TMA) can be defined at a histopathological level or as a clinical syndrome. Histologically, there is thrombus formation affecting small or larger vessels. Thrombi vary in their constituents depending on the underlying cause of the TMA. For example, the thrombi may contain fibrin-platelets (in disseminated intravascular coagulation [DIC] or heparin-induced thrombocytopenia [HIT]), endothelia cells with inflammatory components (as in autoimmune conditions such as lupus or scleroderma), and cancer cells (in malignancy-associated TMA). Von Willebrand factor (VWF)-platelet thrombi are specific in thrombotic thrombocytopenic purpura (TTP) and endothelia damage with fibrinoid necrosis in hemolytic uremic syndrome (HUS). Both HUS and TTP are not typically associated with an inflammatory histological component.

Clinically, TMA is defined by microangiopathic hemolytic anemia and thrombocytopenia (MAHAT). Specifically, there is anemia and a reduced platelet count, with fragmentation and often polychromasia on the blood film. Confirmation of an underlying hemolytic process includes an elevated lactate dehydrogenase (LDH), reticulocytosis, and a low/absent haptoglobin. A direct antiglobulin test should be negative.¹ 

It is important to differentiate TTP and HUS in patients presenting with MAHAT as quickly as possible, as initiation of disease-specific treatment has a critical impact on outcome (Figure 1).² 

This includes congenital and immune-mediated TTP (cTTP and iTTP) and infection-associated or complement-mediated HUS (IA-HUS and CM-HUS). Without treatment, mortality in TTP is 90%. The majority of cases are acquired and immune mediated, and presenting symptoms associated with MAHAT include neurology (such as stroke, transient ischemic attacks, migraines, and seizures), renal impairment, abdominal pain, and cardiac involvement. Cardiac TTP is often less obvious clinically and defined by a raised troponin at presentation.³  A diagnosis of TTP is confirmed by a severe deficiency in ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13), the metalloprotease specifically associated with a diagnosis of TTP.⁴ 

IA-HUS has typically been associated with Shiga toxin-producing Escherichia coli (STEC), typically serotype O157 but also other non-O157 serotypes, as well as Shiga toxin-secreting strains, including Shigella and Campylobacter. It results from ingestion of infected water or food, in animals or person to person. The median incubation period is 3 to 4 days. The toxin acts on vascular endothelium, specifically binding to glycolipid glotriaosylceramide (Gb3) receptors, particularly in the glomerulus of the kidney and, therefore, associated with acute kidney injury. Cases may present with bloody diarrhea, but the TMA usually presents 4 to 7 days after this has resolved. Children are more commonly affected with a mortality of 5% to 10%.⁵  A small percentage of cases of IA-HUS are caused by Streptococcus pneumoniae, which are more likely to be associated with pneumonia and may lead to multi-organ failure. CM-HUS presents with similar laboratory features as IA-HUS and may have diarrhea, but not necessarily bloody. Clinically, an infectious precipitant may need excluding. A viral or bacterial trigger or vaccination may be associated with presentation. However, dysregulation of complement is the underlying defect, although identification of a genetic or acquired abnormality can only be confirmed in 60% to 70% of cases. Mortality has been up to 25% with high rates of end-stage renal failure within 1 year.⁶ 

Despite the urgency of diagnosis and treatment of TTP and HUS (Table 1), there are a number of medical conditions that may present with TMA. The most common is DIC.⁷  This can be precipitated by severe sepsis or trauma, for example. Monitoring the laboratory parameters within scoring systems for DIC correlates with clinical features and outcomes. In particular, thrombocytopenia, which is often the first and most sensitive sign of DIC, is present in >90% of cases with 50% of cases having a platelet count <50 × 109/L. A reduced platelet count is associated with increased thrombin formation and amplified fibrinlytic activity, associated with raised D-Dimers. There is variability in derangement of the coagulation screen in 60% to 70% of cases (PT [prothrombin time] and/or APTT [activated partial thromboplastin time] may be increased, but they may be normal or indeed shortened). Fibrinogen may be reduced, but it is less commonly below the normal laboratory range unless there is very severe DIC. The sequential changes in these parameters may be more helpful in confirming a diagnosis of DIC.⁸  The International Society of Thrombosis and Haemostasis scoring system for DIC is >90% sensitive and specific and predictive of mortality. This was confirmed comparing other scoring systems.⁹  A normal coagulation screen, thrombocytopenia, raised D-Dimers, and fragmentation on a blood film require exclusion of DIC from other TMAs.

An uncommon but eminently treatable cause of TMA is vitamin deficiency, in particular severe B12 deficiency. While there is anemia, raised reticulocytes, and thrombocytopenia, the blood film findings are not specifically in keeping with MAHA. The laboratory parameters at presentation may be identical to TTP, and plasma exchange (PEX) has been initiated in some cases.¹⁰  PEX can be stopped and patients respond promptly to B12 injections. Further follow-up to exclude pernicious anemia with intrinsic factor antibodies or celiac disease is suggested. Therefore, it is proposed that B12 and folate levels are undertaken as part of routine screening samples for acute TMAs.

TMA may be the presenting feature of an underlying cancer, either in undiagnosed disease or in association with metastases. Furthermore, chemotherapeutic medications have been associated with TMA, characterizing a drug-associated HUS (DA-HUS). One of the largest reviews of cancer-associated microangiopathy confirms the majority of cases are solid tumor malignancies, but hematological cancers such as lymphoma make up approximately 8% of all cases. Gastric, lung, breast, and prostate, primarily adenocarcinoma, are the most likely diagnoses. Respiratory symptoms were more commonly featured (not normally seen in TTP or HUS), with an increase in DIC⁷  noted. Treatment with antitumor therapy was associated with improved survival, and there was no role for plasma therapy.¹¹  Distinguishing cancer-associated TMA from TTP may be difficult initially, but the former is more likely to have respiratory symptoms (70% of cases) and bone pain at presentation and have an inadequate response to PEX.¹²  The TMA picture is the result of microthrombi, which may include tumor-associated thrombi or bone marrow involvement by the underlying cancer. Review of the peripheral blood smear and an early bone marrow aspirate and trephine may help expedite the underlying cancer diagnosis.

Diagnosis is important, as there is no beneficial role for PEX, steroids, or other immunosuppressive used in TTP; but, the use of platelet transfusions for severe thrombocytopenia, normally withheld in TTP, would be appropriate. An ADAMTS13 activity level not demonstrating severe deficiency should point to consideration of another TMA, including cancer.¹³  A difficult scenario is the differential among cancer/chemotherapy/CM-HUS, none of which is associated with severe ADAMTS13 deficiency. Furthermore, chemotherapeutic medications have been associated with TMA, typically a DA-HUS picture. There is no current evidence for the use of eculizumab in malignancy-associated TMA. However, there have been case reports/small series demonstrating a possible beneficial effect of complement inhibitor therapy for chemotherapeutic agents causing a TMA.¹⁴ 

Transplant-associated TMA (TA-TMA) may affect either solid organ or hematopoietic stem cell transplant (HSCT) patients. The underlying pathology is endothelial damage,¹⁵  and it may be the result of the prior conditioning therapy, HLA-mismatched transplants, and calceneurin inhibitors used to prevent rejection. It is important to consider an additional underlying infection such as adenovirus; indeed, nearly 50% of patients at post mortem relating to HSCT had evidence of viremia, with some cases only diagnosed at post mortem.¹⁶  The effect of systemic viral infections on already damaged endothelia, from treatment or complications of the transplant, may lead to laboratory features of a TMA.

PEX is not of benefit and treatment is based on symptomatic support. Mortality is raised in such scenarios, and parameters associated with increased mortality include proteinuria, raised LDH, and hypertension.¹⁷  More recently, inherited or acquired defect in complement have been identified and thought to compound the endothelial damage.¹⁸  The use of complement inhibitor therapy has been associated with positive results in some cases/small series.^14,19,20 

Drugs are a rare but important cause of TMA (Table 2).²¹  The most definitive cause of DA-TTP was with ticlopodine.²²  DA-TMAs maybe idiosyncratic (eg, quinine) or dose dependent (eg, IFN).²³  In many cases, the clinical presentation primarily involves severe hypertension with principally a renal element in conjunction with MAHAT. Intravenous abuse of Opana ER (oxymorphone hydrochloride) presents with TMA associated with acute renal injury as well as cardiac and retinal ischemia.²⁴  Other drug-related causes for TMA include chemotherapies^21,25  and therapies used in nonmalignant conditions.^26,27 

Other than stopping the offending medication, therapy remains difficult. PEX is often undertaken but of uncertain utility, as only a small proportion are associated with antibodies to ADAMTS13.²⁸  The pathophysiology of DA-TMAs is varied and can be due to a direct effect on endothelial cells that may be seen with an accumulation of drug dosing, such as with gemcitabine. Alternative mechanisms include antibodies, for example, to red cells or platelets, such as with oxaliplatin.²⁹  Drugs targeting specific proteins, such as vascular endothelial growth factor (VEGF) inhibitors, cause hypertension and microthrombi formation exclusive to glomerular capillaries.^25,30 

Connective tissue diseases (CTDs) may present with a TMA. Acute scleroderma is differentiated by clinical features and may present with or without severe renal involvement, mimicking TTP or HUS.³¹ 

Other conditions include systemic lupus erythematosus (SLE), antiphospholipid syndrome, or vasculitides, such as lupus nephritis or Goodpasture’s syndrome; exclusion of an underlying connective tissue disease requires confirmation by relevant autoantibodies. Caution is required as there are a number of patients with TMAs who have a positive autoantibody screen, especially antinuclear antibody, but do not have SLE.³²  Underlying autoimmune disease needs to be excluded for both TTP and HUS presentations.

Specific infections may be associated with a TMA presentation. HIV may present with TTP, severe ADAMTS13 deficiency, and an immune-mediated pathogenesis.³³  With the advent of highly active antiretroviral therapies (HAARTs), the incidence of TMAs with a primary renal presentation is significantly decreased, and such presentations are more likely in patients with untreated or resistant HIV disease.³⁴  Many other infections may present with a TMA picture, but they require a careful assessment of history, microbiology, and virology. Examples include Dengue fever,³⁵  cytomegalovirus, and tuberculosis. Others like influenza may be associated with the precipitation of TTP or CM-HUS.³⁶ 

Malignant hypertension may mimic presentation of CM-HUS; indeed, it may be difficult to differentiate it from CM-HUS. Endothelial dysfunction may result in hypertension associated with CM-HUS. However, hypertension causing TMA is thought to result from high intra-arterial pressures causing endothelial damage and then secondary TMA, rather than complement overactivation causing endothelial injury and hypertension (Figure 2).³⁷ 

Pregnancy-associated TMAs include specific conditions, eg, TTP or HUS precipitated by pregnancy, or TMAs particular to pregnancy, such as pre-eclampsia (PET), haemolysis, elevated liver enzymes, and low platelets, or acute fatty liver of pregnancy. Differentiating TTP or HUS during pregnancy may be difficult as both conditions can be associated with hypertension, proteinuria, and interuterine growth retardation. TTP can present at any stage during pregnancy, but most commonly it does so in the third trimester. Many cases have been associated with late-onset congenital TTP.³⁸  CM-HUS is thought to be most common in the postpartum period. However, as our understanding of complement-mediated disorders increases, those identified during pregnancy, similar to cTTP, will undoubtedly increase also.³⁹  Consideration for TTP and CM-HUS should be given in patients with thrombocytopenia, particularly with a platelet count <50 × 10⁹/L or a decreasing platelet count. LDH is normal throughout pregnancy, and it is a useful parameter in the consideration of pregnancy-induced TMAs. The incidence of pregnancy-associated TMAs may be greater than originally anticipated. Indeed, 5% of all cases of pregnant women with a platelet count <75 × 10⁹/L during pregnancy have been identified as congenital TTP.⁴⁰  TMAs should be considered as a possible cause of intrauterine fetal death, particularly in the second trimester, and maternal blood counts should be checked. A role for complement in the pathogenesis of pregnancy-associated TMAs³⁹  is suggested in some cases of severe PET⁴¹  and, to a lesser extent, HELLP.^42,43 

There are a collective group of laboratory tests that should be considered in patients with a presentation of TMA. These help not only to elucidate the underlying cause but also to define secondary cases or associated precipitating factors (Table 3).

Acute recognition of TMAs remains a clinical diagnosis. However, the availability of commercial ADAMTS13 assays means confirmation of the diagnosis can be available in real time. ADAMTS13 activity levels <10% is in keeping with TTP.¹  ADAMTS13 activity levels between 10% and 20% require exclusion of the presence of anti-ADAMTS13 antibodies. ADAMTS13 activity levels >10% are suggested to be required for the consideration of CM-HUS.⁴⁴  However, it is very uncommon for HUS cases to have ADAMTS13 activity levels this low, and levels are typically within the middle or normal laboratory range.⁴⁵  Any reduction in ADAMTS13 levels is the result of consumption of ADAMTS13 due to raised VWF levels. A reduction in ADAMTS13 can be documented in severe sepsis-associated DIC, with an increase in ultra large (UL) VWF multimers and related to the risk of renal failure.⁴⁶  This may be seen in a number of physiological and disease states.⁴⁷ 

Due to the disparity in availability of ADAMTS13 assays, a number of scoring systems have been suggested to help differentiate TTP from CM-HUS and other TMAs. Platelet count >30 × 10⁹/L and serum creatinine level >150 to 200 µmol/L may exclude the likelihood of severely deficient ADAMTS13 activity.⁴⁴  From the UK TTP registry, which provides ADAMTS13 activity analysis for all cases of TMA, the median platelet and creatinine results confirm these findings. This can be useful; however, basing therapy on these cutoffs can be detrimental. One-third of HUS cases had a platelet count <30 × 10⁹/L and ∼20% had creatinine levels <150 µmol/L, while 17% of TTP cases had a platelet count >30 × 10⁹/L and 15% had a creatinine >150 µmol/L.⁴⁵  The platelets, lysis, active cancer, stem cell or solid organ transplant, MCV, INR, and creatinine (PLASMIC) score has been suggested as a rapid assessment, in conjunction with clinical assessment, to differentiate TTP due to severe ADAMTS13 deficiency (Table 4).⁴⁸  Scores of 0 to 4 were associated with low risk of severe ADAMTS13 deficiency (0% to 4% of patients), a score of 5 represented intermediate risk (5% to 24% of patients), and scores of 6 to 7 indicated high risk (62% to 82% of patients). Despite these assessments, definitive confirmation of TTP versus another TMA requires ADAMTS13 activity analysis.

The mainstay of treatment of TMAs is PEX. This should be undertaken as soon as the diagnosis is considered, as TTP and HUS are life-threatening disorders. The benefit of PEX has been confirmed in a randomized study.⁴⁹  PEX should be continued to remission in TTP patients (clinical response after cessation of PEX and maintained for greater than 30 days, associated with normalization of ADAMTS13 activity); but, in other TMAs, it would depend on response and results of further laboratory investigations. Typically, steroids are started in conjunction with PEX for iTTP patients; however, they should be used with caution in other situations, such as for HUS or scleroderma, not least because of associated hypertension at presentation.

The use of PEX is not without its risks, for example, from central line insertion, infection, citrate toxicity, and reactions to plasma.⁵⁰  However, many of the investigations to identify the cause of the TMA may be available within 24 to 48 hours from admission, and PEX can be stopped if another diagnosis is suggested for which PEX has no benefit.

If iTTP is confirmed, proceeding to further immunosuppressive therapy, such as anti-CD20 monoclonal antibody therapy, should be considered. Its early use is based on reducing time to remission, less time in the hospital, and reduced need for plasma.⁵¹  It is also established at reducing the risk of relapse, both in the short and longer terms, based on a reduction of anti-ADAMTS13 immunoglobulin G antibodies and increased ADAMTS13 activity.^51,52  Anti-CD20 therapy takes a median of 10 days to achieve remission. Therefore, there is the high-risk acute period that, until recently, remained precarious, while the platelet counts increase. The use of the nanobody, caplacizumab, which prevents platelet binding to VWF, is associated with a faster increment of platelets, following initiation in the acute presentation. It bridges to allow the effect of immunosuppressive therapy on ADAMTS13 autoantibodies.⁵³  Caplacizumab results in acquired von Willebrand disease, and, therefore, it has a potential bleeding phenotype. TTP is a prothrombotic disorder, in which excess bleeding is not usually documented. However, caution is advised for non-TTP cases before initiation, particularly as there is often a severe thrombocytopenia at presentation.

The role of PEX in HUS has, overall, not been confirmed as demonstrating a benefit. However, PEX in the acute presentation is associated with an improvement in hematology parameters and often stabilization or improvement of renal impairment.⁵⁴  Certainly longer term use in the past was associated with recurrent exacerbations. In the current era, the C5 complement inhibitor eculizumab has revolutionized CM-HUS patient outcomes and care. From the pivotal clinical trials, there was 100% normalization of hematology parameters and more than 80% of patients were able to come off dialysis.⁵⁵  Further review confirmed that the earlier the therapy was initiated, the better the outcome, particularly with respect to significant improvements in renal function.⁵⁶ 

Discussion concentrates on ensuring the precise diagnosis that drives the appropriate and relevant therapy. Given the diagnosis of CM-HUS is clinical, a minimum requirement and assessment are suggested before starting eculizumab therapy. Exclusion of an autoimmune diagnosis, eg, SLE and virology including HIV. Normal-sized kidneys on abdominal ultrasound and no features of chronic changes of hypertension on ophthalmic assessment. ADAMTS13 activity >20% excludes a diagnosis of TTP. This is the minimum investigative set of investigations before contemplating complement inhibitor therapy.

The use of immunosuppressives or complement inhibitor therapy may be relevant in other TMAs, eg, autoimmune disease and TA-TMA, respectively. It may also be impossible to exclude TTP or CM-HUS, for example, in malignant hypertension, and a trial of complement inhibitor therapy may be required. Furthermore, dual treatment may be required for those cases of TTP or CM-HUS that have an obvious trigger; for example, HIV infection requires PEX, immunosuppressive therapy to clear ADAMTS13 autoantibodies, and also HAART to reduce the viral load and aid remission. With intensified investigation of complement dysregulation in disease pathogenesis aside from CM-HUS, the rationale and benefit of complement inhibitor therapy in other TMAs are increasing.¹⁴ 

In conclusion, TTP and HUS are important life-threatening TMAs that must be differentiated from other conditions resulting in a TMA. As response is time critical, PEX should be initiated as soon as the diagnoses are considered. Clinical assessments in conjunction with further investigations, including ADAMTS13 assays, are imperative to ensure the precise treatment pathways to achieve remission. For TTP this includes immunosuppression such as rituximab, and for CM-HUS it includes eculizumab. The aim is to reduce mortality and improve long-term mobility.

Marie Scully, Department of Haematology, University College of London Hospitals, 235 Euston Rd, London NW1 2BE, United Kingdom. e-mail: m.scully@ucl.ac.uk.