The impact of transfusing fresher vs older red blood cells (RBCs) on patient-important outcomes remains controversial. Two recently published large trials have provided new evidence. We summarized results of randomized trials evaluating the impact of the age of transfused RBCs. We searched MEDLINE, EMBASE, CINAHL, the Cochrane Database for Systematic Reviews, and Cochrane CENTRAL for randomized controlled trials enrolling patients who were transfused fresher vs older RBCs and reported outcomes of death, adverse events, and infection. Independently and in duplicate, reviewers determined eligibility, risk of bias, and abstracted data. We conducted random effects meta-analyses and rated certainty (quality or confidence) of evidence using the GRADE approach. Of 12 trials that enrolled 5229 participants, 6 compared fresher RBCs with older RBCs and 6 compared fresher RBCs with current standard practice. There was little or no impact of fresher vs older RBCs on mortality (relative risk [RR], 1.04; 95% confidence interval [CI], 0.94-1.14; P = .45; I(2) = 0%, moderate certainty evidence) or on adverse events (RR, 1.02; 95% CI, 0.91-1.14; P = .74; I(2) = 0%, low certainty evidence). Fresher RBCs appeared to increase the risk of nosocomial infection (RR, 1.09; 95% CI, 1.00-1.18; P = .04; I(2) = 0%, risk difference 4.3%, low certainty evidence). Current evidence provides moderate certainty that use of fresher RBCs does not influence mortality, and low certainty that it does not influence adverse events but could possibly increase infection rates. The existing evidence provides no support for changing practices toward fresher RBC transfusion.

Immunoassays are essential in the workup of patients with suspected heparin-induced thrombocytopenia. However, the diagnostic accuracy is uncertain with regard to different classes of assays, antibody specificities, thresholds, test variations, and manufacturers. We aimed to assess diagnostic accuracy measures of available immunoassays and to explore sources of heterogeneity. We performed comprehensive literature searches and applied strict inclusion criteria. Finally, 49 publications comprising 128 test evaluations in 15 199 patients were included in the analysis. Methodological quality according to the revised tool for quality assessment of diagnostic accuracy studies was moderate. Diagnostic accuracy measures were calculated with the unified model (comprising a bivariate random-effects model and a hierarchical summary receiver operating characteristics model). Important differences were observed between classes of immunoassays, type of antibody specificity, thresholds, application of confirmation step, and manufacturers. Combination of high sensitivity (>95%) and high specificity (>90%) was found in 5 tests only: polyspecific enzyme-linked immunosorbent assay (ELISA) with intermediate threshold (Genetic Testing Institute, Asserachrom), particle gel immunoassay, lateral flow immunoassay, polyspecific chemiluminescent immunoassay (CLIA) with a high threshold, and immunoglobulin G (IgG)-specific CLIA with low threshold. Borderline results (sensitivity, 99.6%; specificity, 89.9%) were observed for IgG-specific Genetic Testing Institute-ELISA with low threshold. Diagnostic accuracy appears to be inadequate in tests with high thresholds (ELISA; IgG-specific CLIA), combination of IgG specificity and intermediate thresholds (ELISA, CLIA), high-dose heparin confirmation step (ELISA), and particle immunofiltration assay. When making treatment decisions, clinicians should be a aware of diagnostic characteristics of the tests used and it is recommended they estimate posttest probabilities according to likelihood ratios as well as pretest probabilities using clinical scoring tools.

Pregnancy in women with sickle cell disease is associated with adverse maternal and neonatal outcomes. Studies assessing the effects of prophylactic red blood cell transfusions on these outcomes have drawn inconsistent conclusions. The objective of this systematic review was to assess the effect of prophylactic compared with on-demand red blood cell transfusions on maternal and neonatal outcomes in women with sickle cell disease. A systematic search of several medical literature databases was conducted. Twelve studies involving 1291 participants met inclusion criteria. The studies had moderate to high risk of bias. Meta-analysis demonstrated that prophylactic transfusion was associated with a reduction in maternal mortality (7 studies, 955 participants; odds ratio [OR], 0.23; 95% confidence interval [CI], 0.06-0.91), vaso-occlusive pain episodes (11 studies, 1219 participants; OR, 0.26; 95% CI, 0.09-0.76), pulmonary complications (9 studies, 1019 participants; OR, 0.25; 95% CI, 0.09-0.72), pulmonary embolism (3 studies, 237 participants; OR, 0.07; 95% CI, 0.01-0.41), pyelonephritis (6 studies, 455 participants; OR, 0.19; 95% CI, 0.07-0.51), perinatal mortality (8 studies, 1140 participants; OR, 0.43; 95% CI, 0.19-0.99), neonatal death (5 studies, 374 participants; OR, 0.26; 95% CI, 0.07-0.93), and preterm birth (9 studies, 1123 participants; OR, 0.59; 95% CI, 0.37-0.96). Event rates for most of the results were low. Prophylactic transfusions may positively impact several adverse maternal and neonatal outcomes in women with sickle cell disease; however, the evidence stems from a relatively small number of studies with methodologic limitations. A prospective, multicenter, randomized trial is needed to determine whether the potential benefits balance the risks of prophylactic transfusions.

Fibrinogen, coagulation factor VII (FVII), and factor VIII (FVIII) and its carrier von Willebrand factor (vWF) play key roles in hemostasis. Previously identified common variants explain only a small fraction of the trait heritabilities, and additional variations may be explained by associations with rarer variants with larger effects. The aim of this study was to identify low-frequency (minor allele frequency [MAF] ≥0.01 and <0.05) and rare (MAF <0.01) variants that influence plasma concentrations of these 4 hemostatic factors by meta-analyzing exome chip data from up to 76,000 participants of 4 ancestries. We identified 12 novel associations of low-frequency (n = 2) and rare (n = 10) variants across the fibrinogen, FVII, FVIII, and vWF traits that were independent of previously identified associations. Novel loci were found within previously reported genes and had effect sizes much larger than and independent of previously identified common variants. In addition, associations at KCNT1, HID1, and KATNB1 identified new candidate genes related to hemostasis for follow-up replication and functional genomic analysis. Newly identified low-frequency and rare-variant associations accounted for modest amounts of trait variance and therefore are unlikely to increase predicted trait heritability but provide new information for understanding individual variation in hemostasis pathways.

Therapy for childhood acute lymphoblastic leukemia (ALL) is associated with 5-year survival rates of ∼90% even after largely eliminating cranial radiation. This meta-analysis assesses the long-term neurocognitive functioning after chemotherapy-only regimens among survivors of childhood ALL. We conducted a systematic review to identify studies that evaluated long-term neurocognitive functioning following treatment of ALL by searching MEDLINE/PubMed, Database of Abstracts of Reviews of Effects, and secondary sources. Studies were included if ALL survivors were in continuous first remission, did not receive any radiation, were at least ≥2 years off therapy or ≥5 years since diagnosis, and were compared with a healthy control group. Weighted mean differences with 95% confidence intervals (CIs) were calculated. Ten nonexperimental studies met all eligibility criteria and included 509 patients and 555 controls. Meta-analysis demonstrated statistically significant moderate impairment across multiple neurocognitive domains evaluated, with intelligence most affected. Significant differences in standard deviation (SD) scores were found for Full Scale intelligence quotient (IQ) (-0.52 SD; 95% CI, -0.68 to -0.37), Verbal IQ (-0.54 SD; 95% CI, -0.69 to -0.40), and Performance IQ (-0.41 SD; 95% CI, -0.56 to -0.27); these SD scores correspond to changes in IQ of 6 to 8 points. Working memory, information processing speed, and fine motor domains were moderately, but statistically significantly, impaired. Meta-analysis of ALL survivors treated without cranial radiation demonstrated significant impairment in IQ and other neurocognitive domains. Patients and their families should be informed about these potential negative effects to encourage surveillance and educational planning. Both preventive and intervention strategies are needed.

A systematic review and meta-analysis of observational studies were conducted to quantify the association between sickle cell disease in pregnancy and adverse maternal and perinatal outcomes. Data sources (Medline, Embase, Maternity and Infant care, Cochrane, Web of Science, Popline) were searched for publications to June 2014. Eligibility criteria included observational studies reporting maternal and perinatal health outcomes in pregnant women with sickle cell disease against a comparative group of pregnant women without sickle cell disease. Twenty-one studies (including 26,349 women with sickle cell disease; 26,151,746 women without sickle cell disease) were eligible for inclusion. Pregnancies in women with HbSS genotype, compared with women without sickle cell disease, were at increased risk of maternal mortality (relative risk [RR], 5.98; 95% confidence interval [CI], 1.94-18.44), preeclampsia (RR, 2.43; 95% CI, 1.75-3.39), stillbirth (RR, 3.94; 95% CI, 2.60-5.96), preterm delivery (RR, 2.21; 95% CI, 1.47-3.31), and small for gestational age infants (RR, 3.72; 95% CI, 2.32-5.98). Meta-regression demonstrated that genotype (HbSS vs HbSC), low gross national income, and high study quality were associated with increased RRs. Despite advances in the management of sickle cell disease, obstetrics, and neonatal medicine, pregnancies complicated by the disease remain associated with increased risk of adverse maternal and perinatal outcomes.

Serum cytokines and chemokines may reflect tumor biology and host response in follicular lymphoma (FL). To determine whether the addition of these biological factors may further refine prognostication, 30 cytokines and chemokines were measured in pretreatment serum specimens from newly diagnosed FL patients (n = 209) and from 400 matched controls. Cytokine levels were correlated with clinical outcome in patients who were observed or received single agent rituximab, or those who received chemotherapy. Correlations with outcome in chemotherapy treated patients were further examined in a separate cohort of 183 South West Oncology Group (SWOG) patients and all patients were then included in a meta-analysis. Six cytokines were associated with outcome in the Molecular Epidemiology Resource (MER) after adjusting for the FL international prognostic index. In patients who were observed or treated with rituximab alone, increased serum IL-12 and interleukin 1 receptor antagonist (IL-1RA) (P = .005 and .02) were associated with a shorter event-free survival. In patients receiving chemotherapy, hepatocyte growth factor, IL-8, IL-1RA, and CXCL9 (P = .015, .048, .004, and .0005) predicted a shorter EFS. When the MER chemotherapy treated patients and SWOG patients were combined in a meta-analysis, IL-2R, IL-1RA, and CXCL9 (P = .013, .042, and .0012) were associated with a poor EFS.

Childhood immune thrombocytopenia (ITP) is a rare autoimmune bleeding disorder. Most children recover within 6 to 12 months, but individual course is difficult to predict. We performed a systematic review and meta-analysis to identify predictors of chronic ITP. We found 1399 articles; after critical appraisal, 54 studies were included. The following predictors of chronic ITP in children, assessed in at least 3 studies, have been identified: female gender (odds ratio [OR] 1.17, 95% confidence interval [CI] 1.04-1.31), older age at presentation (age ≥11 years; OR 2.47, 95% CI 1.94-3.15), no preceding infection or vaccination (OR 3.08, 95 CI 2.19-4.32), insidious onset (OR 11.27, 95% CI 6.27-20.27), higher platelet counts at presentation (≥20 × 10(9)/L: OR 2.15, 95% CI 1.63-2.83), presence of antinuclear antibodies (OR 2.87, 95% 1.57-5.24), and treatment with a combination of methylprednisolone and intravenous immunoglobulin (OR 2.67, 95% CI 1.44-4.96). Children with mucosal bleeding at diagnosis or treatment with intravenous immunoglobulin alone developed chronic ITP less often (OR 0.39, 95% CI 0.28-0.54 and OR 0.71, 95% CI 0.52-0.97, respectively). The protective effect of intravenous immunoglobulin is remarkable and needs confirmation in prospective randomized trials as well as future laboratory studies to elucidate the mechanism of this effect.

Plasminogen is the precursor of the serine protease plasmin, a central enzyme of the fibrinolytic system. Plasma levels of plasminogen vary by almost 2-fold among healthy individuals, yet little is known about its heritability or genetic determinants in the general population. In order to identify genetic factors affecting the natural variation of plasminogen levels, we performed a genome-wide association study and linkage analysis in a sample of 3456 young healthy individuals who participated in the Genes and Blood Clotting Study (GABC) or the Trinity Student Study (TSS). Heritability of plasminogen levels was 48.1% to 60.0%. Tobacco smoking and female sex were associated with higher levels of plasminogen. In the meta-analysis, 11 single-nucleotide polymorphisms (SNPs) in 2 regions reached genome-wide significance (P < 5.0E-8). Of these, 9 SNPs were near the PLG or LPA genes on Chr6q26, whereas 2 were on Chr19q13 and 5' upstream of SIGLEC14. These 11 SNPs represented 4 independent signals and collectively explained 6.8% of plasminogen level variation in the study populations. The strongest association was observed for a nonsynonymous SNP in the PLG gene (R523W). Individuals bearing an additional copy of this allele had an average decrease of 13.4% in plasma plasminogen level.

Vitamin K antagonists (VKAs) have been the standard of care for treatment of thromboembolic diseases. Target-specific oral anticoagulants (TSOACs) have been developed and found to be at least noninferior to VKAs with regard to efficacy, but the risk of bleeding with TSOACs remains controversial. We performed a systematic review and meta-analysis of phase-3 randomized controlled trials (RCTs) to assess the bleeding side effects of TSOACs compared with VKAs in patients with venous thromboembolism or atrial fibrillation. We searched MEDLINE, EMBASE, and Cochrane Central Register of Controlled Trials; conference abstracts; and www.clinicaltrials.gov with no language restriction. Two reviewers independently performed study selection, data extraction, and study quality assessment. Twelve RCTs involving 102 607 patients were retrieved. TSOACs significantly reduced the risk of overall major bleeding (relative risk [RR] 0.72, P < .01), fatal bleeding (RR 0.53, P < .01), intracranial bleeding (RR 0.43, P < .01), clinically relevant nonmajor bleeding (RR 0.78, P < .01), and total bleeding (RR 0.76, P < .01). There was no significant difference in major gastrointestinal bleeding between TSOACs and VKAs (RR 0.94, P = .62). When compared with VKAs, TSOACs are associated with less major bleeding, fatal bleeding, intracranial bleeding, clinically relevant nonmajor bleeding, and total bleeding. Additionally, TSOACs do not increase the risk of gastrointestinal bleeding.

In the last 4 years, 6 phase 3 trials including a total of 27,023 patients with venous thromboembolism (VTE) compared a direct oral anticoagulant (DOAC) with vitamin K antagonists (VKAs). To aid the clinician in assessing the amount of information, we address frequently raised clinical questions in a review of combined trial results. We included the phase 3 trials that compared dabigatran etexilate, rivaroxaban, apixaban, or edoxaban with VKA therapy in patients with acute symptomatic VTE. Recurrent VTE occurred in 2.0% of DOAC recipients compared with 2.2% in VKA recipients (relative risk [RR] 0.90, 95% confidence interval [CI] 0.77-1.06). Treatment with a DOAC significantly reduced the risk of major bleeding (RR 0.61, 95% CI 0.45-0.83). In parallel, intracranial bleeding, fatal bleeding, and clinically relevant nonmajor bleeding occurred significantly less in DOAC recipients. The efficacy and safety of DOACs were consistent in patients with pulmonary embolism, deep venous thrombosis, a body weight ≥100 kg, moderate renal insufficiency, an age ≥75 years, and cancer. In conclusion, DOACs and VKAs have similar efficacy in the treatment of acute symptomatic VTE, a finding that is consistent in key clinical subgroups. Treatment with a DOAC significantly reduces the risks of major bleeding.

Thrombin, the major enzyme of the hemostatic system, is involved in biological processes associated with several human diseases. The capacity of a given individual to generate thrombin, called the thrombin generation potential (TGP), can be robustly measured in plasma and was shown to associate with thrombotic disorders. To investigate the genetic architecture underlying the interindividual TGP variability, we conducted a genome-wide association study in 2 discovery samples (N = 1967) phenotyped for 3 TGP biomarkers, the endogenous thrombin potential, the peak height, and the lag time, and replicated the main findings in 2 independent studies (N = 1254). We identified the ORM1 gene, coding for orosomucoid, as a novel locus associated with lag time variability, reflecting the initiation process of thrombin generation with a combined P value of P = 7.1 × 10(-15) for the lead single nucleotide polymorphism (SNP) (rs150611042). This SNP was also observed to associate with ORM1 expression in monocytes (P = 8.7 × 10(-10)) and macrophages (P = 3.2 × 10(-3)). In vitro functional experiments further demonstrated that supplementing normal plasma with increasing orosomucoid concentrations was associated with impaired thrombin generation. These results pave the way for novel mechanistic pathways and therapeutic perspectives in the etiology of thrombin-related disorders.

A 35-year-old woman with recurrent severe placenta-mediated pregnancy complications in her 2 pregnancies asks: Will low-molecular-weight heparin help prevent recurrent placenta-mediated pregnancy complications in my next pregnancy? We performed a meta-analysis of randomized controlled trials (RCTs) comparing low-molecular-weight heparin (LMWH) vs no LMWH for the prevention of recurrent placenta-mediated pregnancy complications. We identified six RCTs that included a total of 848 pregnant women with prior placenta-mediated pregnancy complications. The primary outcome was a composite of pre-eclampsia (PE), birth of a small-for-gestational-age (SGA) newborn (<10th percentile), placental abruption, or pregnancy loss >20 weeks. Overall, 67 (18.7%) of 358 of women being given prophylactic LMWH had recurrent severe placenta-mediated pregnancy complications compared with 127 (42.9%) of 296 women with no LMWH (relative risk reduction, 0.52; 95% CI, 0.32 to 0.86; P = .01; I(2), 69%, indicating moderate heterogeneity). We identified similar relative risk reductions with LMWH for individual outcomes, including any PE, severe PE, SGA <10th percentile, SGA <5th percentile, preterm delivery <37 weeks, and preterm delivery <34 weeks with minimal heterogeneity. LMWH may be a promising therapy for recurrent, especially severe, placenta-mediated pregnancy complications, but further research is required.

Previous studies of anemia epidemiology have been geographically limited with little detail about severity or etiology. Using publicly available data, we estimated mild, moderate, and severe anemia from 1990 to 2010 for 187 countries, both sexes, and 20 age groups. We then performed cause-specific attribution to 17 conditions using data from the Global Burden of Diseases, Injuries and Risk Factors (GBD) 2010 Study. Global anemia prevalence in 2010 was 32.9%, causing 68.36 (95% uncertainty interval [UI], 40.98 to 107.54) million years lived with disability (8.8% of total for all conditions [95% UI, 6.3% to 11.7%]). Prevalence dropped for both sexes from 1990 to 2010, although more for males. Prevalence in females was higher in most regions and age groups. South Asia and Central, West, and East sub-Saharan Africa had the highest burden, while East, Southeast, and South Asia saw the greatest reductions. Iron-deficiency anemia was the top cause globally, although 10 different conditions were among the top 3 in regional rankings. Malaria, schistosomiasis, and chronic kidney disease-related anemia were the only conditions to increase in prevalence. Hemoglobinopathies made significant contributions in most populations. Burden was highest in children under age 5, the only age groups with negative trends from 1990 to 2010.

Acute lymphoblastic leukemia (ALL) is the major pediatric cancer diagnosed in economically developed countries with B-cell precursor (BCP)-ALL, accounting for approximately 70% of ALL. Recent genome-wide association studies (GWAS) have provided the first unambiguous evidence for common inherited susceptibility to BCP-ALL, identifying susceptibility loci at 7p12.2, 9p21.3, 10q21.2, and 14q11.2. To identify additional BCP-ALL susceptibility loci, we conducted a GWAS and performed a meta-analysis with a published GWAS totaling 1658 cases and 4723 controls, with validation in 1449 cases and 1488 controls. Combined analysis identified novel loci mapping to 10p12.2 (rs10828317, odds ratio [OR] = 1.23; P = 2.30 × 10(-9)) and 10p14 marked by rs3824662 (OR = 1.31; P = 8.62 × 10(-12)). The single nucleotide polymorphism rs10828317 is responsible for the N215S polymorphism in exon 7 of PIP4K2A, and rs3824662 localizes to intron 3 of the transcription factor and putative tumor suppressor gene GATA3. The rs10828317 association was shown to be specifically associated with hyperdiploid ALL, whereas the rs3824662-associated risk was confined to nonhyperdiploid non-TEL-AML1 + ALL. The risk allele of rs3824662 was correlated with older age at diagnosis (P < .001) and significantly worse event-free survivorship (P < .0001). These findings provide further insights into the genetic and biological basis of inherited genetic susceptibility to BCP-ALL and the influence of constitutional genotype on disease development.

A whole-genome approach was used to investigate the genetic determinants of cytarabine-induced cytotoxicity. We performed a meta-analysis of genome-wide association studies involving 523 lymphoblastoid cell lines (LCLs) from individuals of European, African, Asian, and African American ancestry. Several of the highest-ranked single-nucleotide polymorphisms (SNPs) were within the mutated in colorectal cancers (MCC) gene. MCC expression was induced by cytarabine treatment from 1.7- to 26.6-fold in LCLs. A total of 33 SNPs ranked at the top of the meta-analysis (P < 10(-5)) were successfully tested in a clinical trial of patients randomized to receive low-dose or high-dose cytarabine plus daunorubicin and etoposide; of these, 18 showed association (P < .05) with either cytarabine 50% inhibitory concentration in leukemia cells or clinical response parameters (minimal residual disease, overall survival (OS), and treatment-related mortality). This count (n = 18) was significantly greater than expected by chance (P = .016). For rs1203633, LCLs with AA genotype were more sensitive to cytarabine-induced cytotoxicity (P = 1.31 × 10(-6)) and AA (vs GA or GG) genotype was associated with poorer OS (P = .015), likely as a result of greater treatment-related mortality (P = .0037) in patients with acute myeloid leukemia (AML). This multicenter AML02 study trial was registered at www.clinicaltrials.gov as #NCT00136084.

Hematopoietic cell transplantation (HCT) and prolonged chemotherapy are standard postremission strategies for adult acute lymphoblastic leukemia in first complete remission, but the optimal strategy remains controversial. There are no randomized trials of allogeneic HCT. In the present study, updated individual patient data were collected and analyzed from studies with information on availability of matched sibling donor (used to mimic randomization) and from randomized trials of autograft versus chemotherapy. Data from 13 studies including 2962 patients, excluding Philadelphia chromosome-positive patients, showed a survival benefit for having a matched sibling donor for patients < 35 years of age (OR = 0.79; 95% CI, 0.70-0.90, P = .0003) but not for those ≥ 35 years of age (OR = 1.01; 95% CI, 0.85-1.19, P = .9; heterogeneity P = .03) because of the higher absolute risk of nonrelapse mortality for older patients. No differences were seen by risk group. There was a trend toward inferior survival for autograft versus chemotherapy (OR = 1.18; 95% CI, 0.99-1.41; P = .06). No beneficial effect of autografting was seen compared with chemotherapy in this analysis. We conclude that matched sibling donor myeloablative HCT improves survival only for younger patients, with an absolute benefit of approximately 10% at 5 years. Improved chemotherapy outcomes and reduced nonrelapse mortality associated with allogeneic HCT may change the relative effects of these treatments in the future.

Myeloproliferative neoplasms (MPNs) are the most common cause of Budd-Chiari syndrome (BCS) and nonmalignant, noncirrhotic portal vein thrombosis (PVT). In this meta-analysis, we determined the prevalence of MPNs and their subtypes as well as JAK2V617F and its diagnostic role in these uncommon disorders. MEDLINE and EMBASE databases were searched. Prevalence of MPNs, JAK2V617F, and MPN subtypes were calculated using a random-effects model. A total of 1062 BCS and 855 PVT patients were included. In BCS, mean prevalence of MPNs and JAK2V617F was 40.9% (95% CI, 32.9%-49.5%) and 41.1% (95% CI, 32.3%-50.6%), respectively. In PVT, mean prevalence of MPNs and JAK2V617F was 31.5% (95% CI, 25.1%-38.8%) and 27.7% (95% CI, 20.8%-35.8%), respectively. JAK2V617F and MPNs were more frequent in BCS compared with PVT (P = .03 and P = .09, respectively). Polycythemia vera was more prevalent in BCS than in PVT (P = .001). JAK2V617F screening in splanchnic vein thrombosis (SVT) patients without typical hematologic MPN features identified MPN in 17.1% and 15.4% of screened BCS and PVT patients, respectively. These results demonstrate a high prevalence of MPNs and JAK2V617F in SVT patients and show differences in underlying etiology between these disorders. Furthermore, these results validate routine inclusion of JAK2V617F in the diagnostic workup of SVT patients.

The 4Ts is a pretest clinical scoring system for heparin-induced thrombocytopenia (HIT). Although widely used in clinical practice, its predictive value for HIT in diverse settings and patient populations is unknown. We performed a systematic review and meta-analysis to estimate the predictive value of the 4Ts in patients with suspected HIT. We searched PubMed, Cochrane Database, and ISI Web of Science for studies that included patients with suspected HIT, who were evaluated by both the 4Ts and a reference standard against which the 4Ts could be compared. Quality of eligible studies was assessed by QUADAS-2 criteria. Thirteen studies, collectively involving 3068 patients, fulfilled eligibility criteria. A total of 1712 (55.8%) patients were classified by 4Ts score as having a low probability of HIT. The negative predictive value of a low probability 4Ts score was 0.998 (95% CI, 0.970-1.000) and remained high irrespective of the party responsible for scoring, the prevalence of HIT, or the composition of the study population. The positive predictive value of an intermediate and high probability 4Ts score was 0.14 (0.09-0.22) and 0.64 (0.40-0.82), respectively. A low probability 4Ts score appears to be a robust means of excluding HIT. Patients with intermediate and high probability scores require further evaluation.