not-yet-known not-yet-known not-yet-known unknown Background The Alfred Step Test Protocol (A-STEP) and feasibly study were previously published. The aim here was to determine the validity of the A-STEP compared to cycle ergometry (CPET) in adults with CF. Methods The A-STEP and CPET were carried out in random order two weeks apart. A wearable, portable metabolic system was used to measure breath-by-breath and minute-by-minute sampling of O 2, CO 2, heart rate and measurement of VO 2. The main outcome measures were VO 2max and HRmax. Results Seven stable state adults (3 male) on CFTR modulator therapy with a mean(SD) and range of : age 38.2(13.4) 26–64 years; height 169.9 (10.9) 149.7-185.3 cm; BMI 22.8 (2.10) 19.5-28 kg/m 2; FEV 1 79.4 (18.9) 38.0-106.0; and FVC 95.1 (16.7) 63.0-114.0 percent predicted (pp) completed both A-STEP and CPET. The VO 2Max had high correlation and good agreement between the A-STEP 31.3 (5.9) and CPET 29.8 (6.2) mL/min/kg, r=0.88. The HR MAX was strongly correlated with the A-STEP 174 (17) bpm and 95.7 (7.4)pp versus 168 (15) bpm and 92.4 (5.3)pp with r=0.92 and 0.86 respectively. The SpO 2Nadir for A-STEP was 91.0 (4.0) and CPET 92.0 (3.3), r=0.82. The VO 2@AT occurred significantly earlier for the CPET at 1021 (260) versus A-STEP 1361 (234) minutes, p<0.05. The VE Max for CPET was 84.1 (18.8) and A-STEP 73.5 (15.8) L/minute, p<0.05. The AWESCORE also ensured baseline stability. The number of levels completed during the A-STEP was 10.7 (12.9) range 9–15. Conclusion The A-STEP is a portable valid surrogate incremental maximal step test to CPET using cycle ergometry.