Knox-Brown B; Respiratory physiology, Cambridge University Hospitals, NHS Foundation Trust, Cambridge, UK.;
Barnes J;Harding C; Fuld J; Sylvester KP;

Experimental physiology [Exp Physiol] 2025 Oct 31.
Date of Electronic Publication: 2025 Oct 31.

There are difficulties with the standardisation of interpretative strategies for peak oxygen uptake (peak INLINEMATH ) related to the quality of reference equations. We aimed to investigate the utility of peak INLINEMATH , a novel method reflecting how far a measured value is from the 1st percentile.
We retrospectively analysed data from patients referred for a cardiopulmonary exercise test (CPET) at Cambridge University Hospital (CUH) and Royal Papworth Hospital (RPH). Data were included from those 18 years or older. We investigated the stability of the 1st percentile overall, then stratified by sex and age group. We calculated the peak INLINEMATH (measured peak INLINEMATH /1st percentile) and investigated its association with all-cause mortality using Cox regression analysis.
Data from 1377 patients were included in the analyses: 590 from CUH (mean age 55.5 (15.4) years, 47% female) and 787 from RPH (mean age 46.1 (15.3) years, 48% female). The 1st percentile value for peak INLINEMATH was 9.5 mL kg ^-1  min ^-1 (95% CI: 8.8-9.9) and was consistent across sex and age groups.
The mean peak INLINEMATH was 2.3 (0.8); it declined with age and was lowest in patients referred for heart transplant. Data on all-cause mortality were available for all patients from CUH. Median follow-up time was 3.8 (2.2-9.6) years, during which time 96 of 590 (16%) patients died. A 1-unit increase in peak INLINEMATH was associated with a 60% reduction in risk of all-cause mortality. We propose the peak INLINEMATH as an alternative means to interpret cardiorespiratory fitness estimates that does not require reference equations.