Shall we go for a walk? A cross-sectional study comparing the 6 min walk test and cardiopulmonary exercise test for assessing cardiorespiratory fitness in de novo kidney transplant recipients

Background and objective Kidney transplant recipients (KTRs) face elevated cardiovascular risk, partly due to reduced cardiorespiratory fitness (CRF). Cardiopulmonary exercise testing (CPET), the gold standard for CRF assessment, plays a central role in clinical evaluation and exercise intervention in other populations, but its use in nephrology is limited by practical constraints. The 6 min walk test (6MWT) offers a simpler alternative, but its correlation and agreement with CPET remain unclear in KTRs. This study aimed to evaluate the correlation and agreement between 6MWT and CPET to assess whether the 6MWT could serve as a viable clinical tool for CRF evaluation post-transplant. Methods This cross-sectional analysis used baseline data of 88 KTRs included in the PHOENIX-Kidney trial ( NCT06260579 ). Participants completed a 3-month post-transplant assessment, including a 6MWT and CPET. Three participants were excluded due to aberrant CPET results requiring further cardiac evaluation, resulting in 85 participants included in the final analysis. Peak oxygen consumption (V̇O₂peak) was measured via CPET and estimated from 6MWT using the Sagat equation. Correlations between 6MWT distance, estimated V̇O₂peak by 6MWT and V̇O₂peak measured by CPET were assessed. Agreement was evaluated using Bland-Altman analysis and intraclass correlation coefficient (ICC). Results A moderate correlation was observed between 6MWT distance and V̇O₂peak measured by CPET (ρ = 0.603, p<0.001), while a strong correlation was found between estimated V̇O₂peak based on the 6MWT and V̇O₂peak measured by CPET (ρ=0.712, p value of <0.001). However, Bland-Altman analysis revealed significant bias and wide limits of agreement, indicating poor agreement between two methods. Consistently, agreement assessed by the ICC was poor (ICC=0.26, 95% CI −0.10 to 0.58). Conclusion Although the 6MWT demonstrates moderate correlations with CPET at the group level, its limited precision for individual prediction highlights the importance of using CPET for accurate assessment of CRF in KTRs.