MM is the leading indication for Autologous Stem Cell Transplantation (ASCT) globally. Although it is a safe procedure, it might be accompanied by significant morbidity leading to ICU admissions. The Hematopoietic Cell Transplantation-specific Comorbidity Index (HCT-CI) is widely used to gauge the risk of treatment-related mortality for patients receiving allografts.

To characterize the outcomes of a large cohort of patients submitted to the first ASCT for MM and to evaluate the HCT-CI as a tool to estimate morbidity for patients with MM undergoing ASCT in a Brazilian single-center retrospective cohort.

We retrospectively reviewed the charts of patients undergoing the first ASCT for MM from January 2017 to December 2022. The HCT-CI was calculated for each patient, and the outcome of interest (admission to the ICU) was assessed. The HCT-CI test performance, dichotomized as non-high-risk versus high-risk parameters, was estimated for sensitivity, specificity, positive likelihood ratio, negative likelihood ratio, positive predictive value, and negative predictive value.

The cohort's median age was 61.6 years, with a similar distribution between genders, 96% obtained at least a partial response before the transplant. The HCT-CI was classified as low-risk in 54.2% of participants, intermediate risk in 35.6%, and high risk in 10.2%. 85% underwent conditioning with 200 mg/m2 melphalan, and 14.1% with 140 mg/m2. The 3-year Overall Survival (OS) was 80.6% (95% CI 75.4‒86.2). The rates of neutrophil and platelet engraftment by D+25 were 98% and 86%, respectively. The transplant-related mortality (TRM) rate was 3.6% (95% CI 2‒6) up to D+60. The cumulative incidence of ICU admission in 30 days was 15% (95% CI 11‒19). The reasons for ICU admission were cardiac arrhythmia (35.2%), acute respiratory failure (7.4%), septic shock (55.6%), and trauma (1%). The average ICU length of stay was 10.8 days, with 74% being discharged alive. The HCT-CI had a sensitivity of 12.96% (95% CI 5.37‒24.9) and specificity of 90.36% (95% CI 86.28‒93.55) in predicting ICU admission. The positive likelihood ratio was 1.34 (95% CI 0.62‒2.96), and the negative likelihood ratio was 0.96 (95% CI 0.86‒1.08). The positive predictive value was 20.59% (95% CI 8.7‒37.9), and the negative predictive value was 84.33% (95% CI 79.72‒88.26), with a calculated accuracy of 77.84% (95% CI 73‒82.18).

Reliable methods to assess patient vulnerability to HSCT are crucial for an informed therapeutic decision. The HCT-CI is often used to assess the risk of morbidity and mortality after transplantation. In our study, the HCT-CI showed low sensitivity and high specificity. Its positive predictive value was only 20%, with 16% of the cohort being misclassified as at risk for the event without experiencing it. Both positive and negative likelihood ratios, which represent the test's ability to correctly influence our thinking about the presence or absence of disease, showed uncertain results and a minimal effect on basic pre-test probability estimates.

The HCT-CI is not a reliable method to predict HSCT-related morbidity. The need to improve a specific risk score for myeloma is highlighted by these findings. Not assessing the HCT-CI as an ordinal variable, potentially missing optimal cut-point identification via ROC analysis, could be a study limitation.