Prognostic value of venous thromboembolism risk scores in patients with severe COVID-19
The International Medical Prevention Registry on Venous Thromboembolism (IMPROVE) and Caprini risk scores were independent predictors of mortality and thrombotic events in patients with severe coronavirus disease (COVID-19).

Key Takeaway

  • Among patients hospitalized for severe COVID-19, IMPROVE and Caprini risk assessment models were both strong and independent predictors of mortality.
  • The two scores also strongly and independently predicted thrombotic events among patients classified at “high risk for venous thromboembolism (VTE)” by either risk assessment model (RAM).

Why This Matters

  • Particular endothelial involvement and high VTE incidence in patients with COVID-19 are highly associated with worse outcomes; however, the optimal way to classify these patients is unclear.
  • This study assessed if VTE RAMs, like IMPROVE and Caprini scores, can provide prognostic information and predict mortality in patients with severe COVID-19.

Study Design

  • The present retrospective, observational study enrolled consecutive patients hospitalized for severe COVID-19 from the NorthShore University Health System (March 12, 2020–July 30, 2020).
  • Inclusion criteria: Patients (≥18 years) with severe acute respiratory syndrome-coronavirus-2 infection confirmed by polymerase chain reaction
  • According to the IMPROVE risk score, patients were classified into low (score 0–1), moderate (score 2–3), and high risk for VTE (score ≥4).
  • According to the Caprini score, patients were classified as follows: Very low (score: 0–2), moderate (score: 3–4), high (score: 5–6), very high (score: 7–8), and the highest risk (score: >8)
  • Outcomes: (1) VTE (deep vein thrombosis or pulmonary embolism) and (2) clinical and mortality outcomes

Key Results

  • In total, 184 patients with severe COVID-19 were included (men, 63.6%; age, 63 [interquartile range: 24–101] years).

IMPROVE score and mortality

  • Moderate and high risk for VTE by IMPROVE score were significantly associated with mortality (hazard ratio [HR] = 2.02; 95% confidence interval [CI]: 1.27–3.21; P = 0.003 vs HR = 2.49; 95% CI: 1.62–3.83; P <0.001), with 87% sensitivity and 63% specificity (area under the curve [AUC] = 0.752, P <0.001).
  • In multivariate Cox’s regression analysis, the association stayed significant for the following:
    • High risk for VTE (HR = 6.22; 95% CI: 3.04–12.71; P <0.001)
    • Moderate risk for VTE (HR = 5.68; 95% CI: 2.93–11.03; P <0.001)
    • Diabetes presence (HR = 1.70; 95% CI: 1.10–2.63; P = 0.016)
    • Hypertension presence (HR = 1.63; 95% CI: 1.02–2.59; P <0.040)
  • In-hospital mortality in patients at low, moderate, and high risk for VTE was 15.1%, 66.7%, and 68.1%, respectively.

Caprini score and mortality

  • High risk for VTE by Caprini score had significant association with mortality (HR = 18.6; 95% CI: 5.87–59.06; P <0.001), with 96% sensitivity and 55% specificity (AUC = 0.843, P <0.001).
  • This association remained significant in multivariate Cox’s regression analysis (HR = 17.6; 95% CI: 5.56–55.96; P <0.001), followed by the presence of diabetes (HR = 1.60; 95% CI: 1.05–2.46; P = 0.029).
  • Patients at very low, moderate, high, very high, and the highest risk had 0%, 6.8%, 44.4%, 60.8%, and 80% mortality rate.

Prediction of thrombotic events

  • In univariate and multivariate analyses, patients at high risk for VTE by IMPROVE and Caprini scores were statistically associated with thrombotic event occurrence (HR = 6.50; 95% CI: 2.72–15.53; P <0.001 and HR = 11.507; 95% CI: 2.697–49.104; P = 0.001, respectively).
  • Those with moderate risk for VTE by IMPROVE score were not statistically related with thrombotic event incidence (HR = 2.82; 95% CI: 0.62–12.75; P = 0.177).
  • The cumulative incidence of VTE for patients with low, moderate, and high risk for VTE was 5.5%, 7.7%, and 27.8%, respectively.
  • Based on Caprini scores, patients classified at very low, moderate, high, very high, and the highest risk had a VTE incidence of 0%, 4.5%, 7.4%, 17.6%, and 28.0%, respectively.


  • The study was observational in nature, had relatively small sample size, and enrolled patients from a single center.
  • Owing to missed routine screenings for VTE or autopsies during the pandemic, it could not be verified whether the deaths were related to VTE.
  • The true number of thrombotic events was probably underestimated.
  • Severity of disease from COVID-19 introduced selection bias, limiting generalizability of results to all patients with COVID-19.
  • Owing to retrospective design, a formal power calculation was not performed.
    1. Paz Rios LH, Minga I, Kwak E, Najib A, Aller A, Lees E, et al. Prognostic value of venous thromboembolism risk assessment models in patients with severe COVID-19. TH Open. 2021;5(2):e211–e219. doi: 10.1055/s-0041-1730293. PMID: 34179684.