This calculator is provided for informational and educational purposes only. It is not intended to replace professional medical advice, diagnosis, or treatment. Always consult with a qualified healthcare professional before making any medical decisions. The results from this calculator should be used as a reference guide only and not as the sole basis for clinical decisions. The PLASMIC score is a clinical prediction tool that estimates probability of severe ADAMTS13 deficiency – it does not diagnose thrombotic thrombocytopenic purpura. All patients with suspected TTP require urgent haematology consultation.
PLASMIC Score Calculator
Calculate the PLASMIC score for thrombotic thrombocytopenic purpura (TTP) risk stratification. Enter seven laboratory and clinical criteria – platelet count, combined haemolysis variable, absence of active cancer, absence of stem cell transplant history, MCV, INR, and creatinine – to predict severe ADAMTS13 deficiency probability and guide therapeutic plasma exchange decisions.
| PLASMIC Score | Risk Category | ADAMTS13 Deficiency Probability | Recommended Action |
|---|---|---|---|
| 0 – 4 | Low Risk | Approximately 4% | Investigate alternative TMA diagnoses (complement-mediated HUS, DIC, drug-induced TMA). Haematology consultation. |
| 5 | Intermediate Risk | Approximately 26% | Urgent haematology consultation. Consider empirical TPE based on clinical features and availability of rapid ADAMTS13 testing. |
| 6 – 7 | High Risk | Approximately 72% | Urgent haematology consultation. Strong consideration of empirical therapeutic plasma exchange (TPE) with FFP replacement while awaiting ADAMTS13 results. |
Probability estimates are pooled from validation studies (Bendapudi et al. 2017, Li et al. 2019 meta-analysis). Individual centre performance may vary. All figures are approximate and should be interpreted in clinical context.
| Letter | Criterion | Threshold for 1 Point | Clinical Rationale |
|---|---|---|---|
| P | Platelet count | Below 30 x10⁹/L | Severe thrombocytopenia reflects massive platelet consumption in TTP microthrombi |
| L | Combined haemolysis variable | Any one of: Reticulocyte count >2.5%, haptoglobin undetectable, indirect bilirubin >2 mg/dL (34 micromol/L) | Confirms active intravascular haemolysis from microangiopathic red cell fragmentation |
| A | Absence of active cancer | No active malignancy present | Malignancy-associated TMA mimics TTP but is independent of ADAMTS13 deficiency |
| S | Absence of stem cell transplant | No prior haematopoietic stem cell transplant | Transplant-associated TMA is a distinct entity driven by endothelial injury, not ADAMTS13 deficiency |
| M | MCV | Below 90 fL | Reflects absence of macrocytosis from chronic comorbidities; TTP patients tend to be otherwise healthy |
| I | INR | Below 1.5 | Near-normal coagulation argues against DIC; in TTP, coagulation factors are largely preserved |
| C | Creatinine | Below 2.0 mg/dL (176 micromol/L) | Severe renal impairment is more characteristic of complement-mediated TMA (atypical HUS) than TTP |
| Entry | Time | PLASMIC Score | Risk Category | ADAMTS13 Probability |
|---|---|---|---|---|
| No scores saved yet. Calculate a score and press Save. | ||||
This PLASMIC score calculator is for educational and clinical decision support purposes only. It does not replace clinical judgment, specialist consultation, or formal diagnostic testing including ADAMTS13 activity measurement. All patients with suspected thrombotic thrombocytopenic purpura require urgent haematology evaluation. Therapeutic plasma exchange decisions must be made by qualified physicians with access to the full clinical picture. Do not use this tool as the sole basis for treatment initiation or withholding.
About This PLASMIC Score Calculator for TTP Risk Stratification
This PLASMIC score calculator is designed for emergency physicians, haematologists, intensivists, and internal medicine clinicians evaluating patients with suspected thrombotic thrombocytopenic purpura (TTP). It calculates the seven-criterion PLASMIC score – covering platelet count, combined haemolysis variable, absence of active cancer, absence of stem cell transplant history, MCV, INR, and serum creatinine – to generate a probability estimate for severe ADAMTS13 deficiency and stratify TTP risk into low, intermediate, and high categories.
The PLASMIC score was derived by Bendapudi and colleagues at Massachusetts General Hospital and published in The Lancet Haematology in 2017. Each criterion is grounded in the pathophysiology of immune-mediated TTP versus alternative thrombotic microangiopathies including complement-mediated HUS, disseminated intravascular coagulation, and malignancy-associated TMA. The calculator accepts creatinine in both mg/dL and micromol/L, reflecting the different unit systems used internationally, with automatic threshold conversion.
Validated across North American, European, and Asian cohorts with an area under the ROC curve of approximately 0.90, the PLASMIC score enables timely therapeutic plasma exchange decisions before ADAMTS13 laboratory results are available – a critical capability given the hours to days required for ADAMTS13 testing at most institutions. The Severity Reference tab provides risk category interpretation with recommended actions; the Clinical Criteria Detail tab explains the rationale behind each of the seven PLASMIC criteria; and the Score History Log enables comparison of serial assessments within a single clinical encounter. This tool is intended to support, not replace, haematology consultation and clinical judgment in all cases of suspected TTP.
PLASMIC Score for TTP – Complete Clinical Guide to ADAMTS13 Deficiency Risk Stratification
Thrombotic thrombocytopenic purpura (TTP) is a life-threatening thrombotic microangiopathy (TMA) that demands rapid clinical recognition and urgent treatment. The PLASMIC score was developed to help clinicians rapidly stratify the probability of severe ADAMTS13 deficiency – the defining biochemical feature of immune-mediated TTP – before confirmatory laboratory results are available. Because ADAMTS13 testing can take days to return, the PLASMIC score enables clinicians to make timely decisions about initiating therapeutic plasma exchange (TPE), which is the cornerstone of TTP management.
This guide explains the PLASMIC score in full: its derivation, each of its seven criteria, how to interpret results, its validation across diverse clinical populations, and how it fits into contemporary TTP management pathways. Whether you are an emergency physician, haematologist, intensivist, or internal medicine practitioner, understanding this tool is essential for managing patients presenting with unexplained thrombocytopenia and microangiopathic haemolytic anaemia (MAHA).
L – Combined haemolysis variable (reticulocyte count >2.5% OR undetectable haptoglobin OR indirect bilirubin >2 mg/dL)
A – Absence of active cancer
S – Absence of stem cell transplant history
M – MCV <90 fL
I – INR <1.5
C – Creatinine <2.0 mg/dL (176 micromol/L)
What Is Thrombotic Thrombocytopenic Purpura?
TTP is a rare but potentially fatal condition characterised by systemic platelet-rich thrombi in small blood vessels throughout the body. These microthrombi form when ultra-large von Willebrand factor (VWF) multimers accumulate in the circulation due to severe deficiency of ADAMTS13, the metalloprotease responsible for cleaving VWF multimers to appropriate size. In immune-mediated TTP – the most common form in adults – autoantibodies directed against ADAMTS13 reduce its activity to below 10% of normal, a threshold defined as severe deficiency.
The resulting platelet-VWF thrombi cause the characteristic features of TTP: thrombocytopenia (from platelet consumption), microangiopathic haemolytic anaemia (from red cell fragmentation as they pass through partially occluded microvasculature), and ischaemic injury to organs – most critically the brain and kidneys. Without treatment, mortality historically exceeded 90%. With prompt therapeutic plasma exchange and immunosuppression, survival rates now exceed 80-90% in most series.
Clinically, the differential diagnosis of TTP encompasses other TMAs including haemolytic uraemic syndrome (HUS), complement-mediated TMA (atypical HUS), disseminated intravascular coagulation (DIC), HELLP syndrome (in pregnancy), and drug-induced TMA. Distinguishing these conditions rapidly is critical because their treatments differ substantially.
The Need for a Clinical Prediction Tool
ADAMTS13 activity testing, the gold standard for diagnosing TTP, requires specialised laboratory equipment and expertise. Most clinical laboratories send samples to reference laboratories, and turnaround times of 24 to 72 hours or more are common. However, TTP is a haematological emergency where delay in initiating TPE is associated with increased organ damage and mortality. This creates a clinical dilemma: when should TPE be started empirically before ADAMTS13 results are available?
Before the PLASMIC score, clinicians relied on clinical gestalt or the classic “pentad” of TTP (fever, thrombocytopenia, MAHA, neurological symptoms, and renal dysfunction). However, the full pentad is present in fewer than 5% of patients at presentation, making it an unreliable diagnostic criterion in isolation. What was needed was a validated, objective scoring system using readily available clinical and laboratory data that could be obtained within hours of presentation.
Derivation and Validation of the PLASMIC Score
The PLASMIC score was derived by Bendapudi and colleagues and published in The Lancet Haematology in 2017. The derivation cohort consisted of patients evaluated for TTP at Massachusetts General Hospital. Using logistic regression analysis, seven variables independently associated with severe ADAMTS13 deficiency were identified from a set of clinical and laboratory parameters available at the time of initial evaluation.
The score was subsequently validated in multiple independent cohorts across North America, Europe, Asia, and Australia. A systematic review and meta-analysis published in 2019 pooled data from over 1,000 patients and confirmed excellent discriminative performance with an area under the receiver operating characteristic curve (AUROC) of approximately 0.90. High scores (6-7) carried a positive likelihood ratio exceeding 6, while low scores (0-4) had a negative likelihood ratio below 0.10, supporting its utility across the spectrum of pre-test probability.
The PLASMIC score was derived at Massachusetts General Hospital and validated in multiple international cohorts. Its AUROC of approximately 0.90 reflects strong discriminative performance for predicting severe ADAMTS13 deficiency (<10% activity) across diverse patient populations worldwide.
Understanding Each PLASMIC Criterion
Each of the seven criteria is worth one point, and the rationale behind each is grounded in the pathophysiology of TTP versus alternative diagnoses.
P – Platelet count below 30 x10⁹/L: Severe thrombocytopenia (below 30 x10⁹/L) is characteristic of TTP, reflecting massive platelet consumption by microthrombi. Other causes of TMA, particularly complement-mediated HUS, may present with less profound thrombocytopenia. A platelet count below this threshold increases the probability of immune-mediated TTP.
L – Combined haemolysis variable: This criterion is satisfied if any one of three markers of haemolysis is present: reticulocyte count above 2.5%, undetectable haptoglobin, or indirect bilirubin above 2 mg/dL (34 micromol/L). These markers confirm active intravascular haemolysis from red cell fragmentation. The combined variable captures haemolysis even when individual markers may not reach threshold.
A – Absence of active cancer: Malignancy-associated TMA is a recognised mimic of TTP. In patients with active cancer, platelet-fibrin thrombi can form via mechanisms independent of ADAMTS13 deficiency – particularly in adenocarcinoma metastases to the microvasculature. Active cancer therefore reduces the probability that severe ADAMTS13 deficiency is the underlying mechanism, and this criterion awards a point only when active cancer is absent.
S – Absence of stem cell transplant history: Haematopoietic stem cell transplant (HSCT) recipients can develop TMA as a complication of the transplant process, graft-versus-host disease, or calcineurin inhibitor toxicity – mechanisms that are largely independent of ADAMTS13 deficiency. The absence of prior HSCT therefore increases the likelihood that the TMA is immune-mediated TTP.
M – MCV below 90 fL: A mean corpuscular volume below 90 fL suggests that pre-existing microcytic anaemia (from iron deficiency, thalassaemia, or anaemia of chronic disease) is not confounding the haematological picture. More importantly, the MCV is a proxy marker: patients with TTP tend to be otherwise healthy individuals who present acutely, without the chronic comorbidities that raise MCV (such as vitamin B12 or folate deficiency). An MCV below 90 fL in this context increases the probability of TTP.
I – INR below 1.5: A normal or near-normal INR argues against consumptive coagulopathy such as DIC, in which coagulation factors are depleted alongside platelets. In TTP, the coagulation cascade is largely intact because the pathology is primarily one of platelet-VWF thrombus formation rather than widespread fibrin deposition. An INR below 1.5 therefore supports TTP over DIC as a diagnosis.
C – Creatinine below 2.0 mg/dL (176 micromol/L): While TTP does cause renal involvement, severe renal impairment is more characteristic of HUS (particularly Shiga toxin-mediated HUS in children and complement-mediated HUS in adults) than of TTP. A creatinine below 2.0 mg/dL increases the probability of TTP over complement-mediated TMA or other nephrotoxic causes.
Score Interpretation and Risk Stratification
The PLASMIC score ranges from 0 to 7, with higher scores corresponding to higher probability of severe ADAMTS13 deficiency:
Score 5: Intermediate Risk (~26% probability)
Score 6-7: High Risk (~72% probability)
These probability estimates have important clinical implications. In a high-risk patient (score 6-7), empirical initiation of TPE while awaiting ADAMTS13 results is strongly supported by the evidence and by guidelines from the American Society of Hematology (ASH) and the British Society for Haematology (BSH). In low-risk patients (score 0-4), alternative diagnoses should be actively pursued before committing to TPE, though clinical context always takes precedence.
Integration into TTP Management Pathways
Guidelines from major haematology societies increasingly incorporate the PLASMIC score into their recommended management algorithms. The ASH 2020 guidelines and BSH 2012 (updated) guidelines both recommend risk stratification tools to guide TPE decisions in suspected TTP. While specific guideline recommendations vary, the general principle is consistent: high PLASMIC scores should prompt urgent haematology consultation and strong consideration of empirical TPE.
TPE works in immune-mediated TTP through two mechanisms: removal of the autoantibody directed against ADAMTS13, and replacement of ADAMTS13 activity via fresh frozen plasma (FFP) used as the replacement fluid. Corticosteroids (typically prednisolone 1 mg/kg/day) are used concurrently for immunosuppression, and rituximab is increasingly used in refractory cases or as a first-line adjunct to prevent relapse.
A PLASMIC score of 6-7 should prompt urgent haematology consultation and strong consideration of empirical therapeutic plasma exchange while awaiting ADAMTS13 results. A score of 0-4 should prompt investigation of alternative TMA diagnoses including complement-mediated HUS, drug-induced TMA, and DIC.
Limitations of the PLASMIC Score
Despite its robust validation, the PLASMIC score has several important limitations that clinicians must recognise.
First, the score predicts severe ADAMTS13 deficiency, not TTP per se. ADAMTS13 deficiency is the biochemical hallmark of immune-mediated TTP, but the score does not directly diagnose TTP – it estimates the probability of the underlying enzymatic deficiency. A confirmed diagnosis requires ADAMTS13 activity below 10% with or without detectable inhibitor.
Second, patients with congenital TTP (Upshaw-Schulman syndrome), caused by homozygous or compound heterozygous mutations in the ADAMTS13 gene rather than autoantibodies, may score differently from immune-mediated TTP patients. Congenital TTP is rare and often presents in childhood or during pregnancy.
Third, the score was derived in a tertiary referral centre with a specific case mix. In populations where alternative TMAs are more prevalent (such as Shiga toxin-mediated HUS in paediatric populations), the positive predictive value may differ.
Fourth, the L criterion (combined haemolysis variable) requires either reticulocyte count or haptoglobin measurement, which may not always be immediately available. Clinicians should obtain these early in the workup of any patient with suspected TMA.
Fifth, the score does not account for clinical features such as neurological symptoms, fever, or the clinical trajectory of the patient. These remain important contextual factors that should inform overall management decisions.
Comparison with Other TMA Risk Tools
The PLASMIC score is the most widely adopted clinical prediction tool for severe ADAMTS13 deficiency globally, but other approaches have been proposed. The French score, developed by Benhamou and colleagues, uses similar variables in a different combination and has been validated primarily in French patient cohorts. Studies comparing the two tools have generally shown comparable performance, though head-to-head comparisons are limited by differences in cohort characteristics.
For complement-mediated TMA (atypical HUS), different clinical and genetic criteria are relevant – PLASMIC score is not designed to diagnose aHUS. The complement-mediated TMA spectrum requires assessment of complement genetics and activity, and eculizumab therapy rather than TPE is the primary treatment.
Special Populations and Considerations
Pregnancy-associated TTP presents particular challenges. HELLP syndrome and pre-eclampsia can mimic TTP, and ADAMTS13 activity can be mildly reduced in normal pregnancy. TTP occurring in pregnancy or the postpartum period may present with atypical features, and the PLASMIC score has been less rigorously validated in this population specifically. Consultation with maternal-fetal medicine specialists and haematologists is essential.
In HIV-infected patients, both TTP and thrombocytopenic conditions related to HIV infection itself must be considered. The PLASMIC score can be applied in this population, though its performance characteristics have not been specifically validated in HIV cohorts.
Critically ill patients in the intensive care unit (ICU) may present with thrombocytopenia and MAHA from multiple causes simultaneously, including sepsis-associated DIC, drug toxicity, and haematological malignancy. The PLASMIC score retains utility in this setting but must be interpreted alongside the full clinical picture.
The PLASMIC score has been validated primarily in general adult hospital populations. In pregnancy, HIV infection, and critically ill patients, its performance may differ, and clinical judgment must supplement the score. Haematology consultation is recommended in all clinically uncertain cases regardless of the PLASMIC score result.
Validation Across Diverse Populations
The PLASMIC score has been validated across multiple continents and ethnic populations. Studies from North America, Europe, Asia, and Australia have demonstrated consistent performance. A 2019 meta-analysis by Li and colleagues pooled data from multiple validation cohorts including patients from the United States, France, the Netherlands, China, and Australia, demonstrating that the score’s discriminative performance (AUROC approximately 0.89-0.92) was maintained across these diverse settings.
Ethnic variation in ADAMTS13 genetics and TTP incidence exists, but the clinical variables comprising the PLASMIC score are pathophysiologically grounded rather than population-specific, contributing to its generalisability. The score performs similarly in Asian, Black, Hispanic, and White patient populations in published series, though larger dedicated population-specific studies would strengthen this evidence base.
Laboratory Units and Global Variations
Clinicians worldwide use different unit systems for laboratory values included in the PLASMIC score. The key conversions are:
- Creatinine: The threshold is 2.0 mg/dL (US) = 176 micromol/L (SI units used in most countries outside the US)
- Indirect bilirubin: 2 mg/dL = 34 micromol/L
- Platelet count: 30 x10⁹/L = 30,000/microlitre = 30 x10³/microlitre (all equivalent)
- MCV: fL (femtolitres) is universal
- Reticulocyte count: 2.5% is a percentage of red cells, reported consistently worldwide
Most modern electronic health records and laboratory information systems display values in locally relevant units. Clinicians should verify the unit system in use at their institution when applying the creatinine and bilirubin thresholds.
Clinical Workflow – Applying the PLASMIC Score
When a patient presents with unexplained thrombocytopenia and clinical or laboratory evidence of haemolytic anaemia, the following stepwise approach integrates the PLASMIC score effectively:
Step 1 – Confirm MAHA: Review the blood film for schistocytes (fragmented red cells). The presence of schistocytes on peripheral blood film is a prerequisite finding for suspecting TTP. Obtain full blood count, reticulocyte count, haptoglobin, LDH, indirect bilirubin, INR, creatinine, and MCV.
Step 2 – Send ADAMTS13: Send an ADAMTS13 activity assay and inhibitor titre immediately. Results take time, but the sample must be collected before any plasma transfusion or TPE to avoid dilution of the autoantibody.
Step 3 – Calculate PLASMIC score: Apply the seven criteria using the results from Step 1. This can be done within 1-2 hours of presentation once laboratory results return.
Step 4 – Risk-stratified management: For high-risk patients (score 6-7), initiate empirical TPE and corticosteroids while awaiting ADAMTS13 results. For intermediate-risk patients (score 5), weigh clinical features and consider haematology consultation. For low-risk patients (score 0-4), continue investigating for alternative diagnoses.
Step 5 – Confirm and adjust: When ADAMTS13 results return, confirm or revise the diagnosis and adjust management accordingly. ADAMTS13 activity below 10% confirms severe deficiency and supports immune-mediated TTP.
The Role of Therapeutic Plasma Exchange
TPE is the most effective treatment for acute immune-mediated TTP and is considered a medical emergency when indicated. Each TPE session exchanges approximately 1-1.5 plasma volumes (40-60 mL/kg), removing autoantibodies and replenishing ADAMTS13. Daily TPE is standard until platelet count normalises and LDH returns to the normal range, typically over 5-10 days in uncomplicated cases.
The decision to initiate TPE empirically – before ADAMTS13 results return – is supported by the high mortality of untreated TTP. A delay of even 24-48 hours can result in irreversible neurological injury or death. For patients with high PLASMIC scores, the benefit of empirical TPE substantially outweighs the procedural risks (line infection, citrate toxicity, allergic reactions to replacement plasma), which are manageable in an experienced centre.
In patients with high PLASMIC scores (6-7), initiating therapeutic plasma exchange empirically while awaiting ADAMTS13 results is recommended by major haematology guidelines. Delay increases the risk of irreversible end-organ damage. Conversely, unnecessary TPE in low-risk patients carries procedural risk and resource burden, reinforcing the value of accurate risk stratification.
Monitoring Response and Relapse
Response to TPE is monitored by daily platelet count and LDH measurement. A rising platelet count and falling LDH indicate treatment response. Most patients achieve platelet count normalisation within 7-14 days of starting TPE. Once the platelet count exceeds 150 x10⁹/L for two consecutive days, TPE can typically be tapered and discontinued.
Relapse occurs in approximately 30-50% of patients with immune-mediated TTP, usually within the first two years. Rituximab (anti-CD20 monoclonal antibody) is increasingly used as first-line therapy alongside TPE and steroids in acute TTP, and has been shown to reduce relapse rates substantially. Long-term follow-up of ADAMTS13 activity enables early detection of relapse before clinical TTP recurs.
Caplacizumab, an anti-VWF nanobody approved in several jurisdictions including Europe and the United States, has also demonstrated efficacy in acute TTP by blocking VWF-platelet interaction. When used alongside TPE and immunosuppression, it accelerates platelet recovery and reduces TTP-related deaths and major thromboembolic events.
Frequently Asked Questions
Conclusion
The PLASMIC score provides clinicians with a validated, rapid, and objective tool for stratifying the probability of severe ADAMTS13 deficiency in patients presenting with suspected TTP. Its seven readily available laboratory criteria can be calculated within hours of presentation, enabling timely risk-stratified management decisions that are central to improving outcomes in this life-threatening haematological emergency.
High PLASMIC scores (6-7) should prompt urgent haematology consultation and strong consideration of empirical therapeutic plasma exchange while ADAMTS13 results are awaited. Low scores (0-4) should redirect the diagnostic focus toward alternative causes of TMA. Intermediate scores require individualised assessment of clinical features and the overall probability of TTP in the specific clinical context.
As with all clinical prediction tools, the PLASMIC score supplements but does not replace clinical judgment. Schistocytes on peripheral blood film, neurological features, the patient’s clinical trajectory, and the pre-test probability of TTP based on the overall presentation all remain essential inputs to management decisions. This calculator is intended to assist qualified healthcare professionals in applying the PLASMIC score and should be used alongside formal clinical evaluation and specialist consultation.