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Abstract Choosing optimal outcome measures maximizes statistical power, accelerates discovery and improves reliability in early-phase trials. We devised and evaluated a modification to a pragmatic measure of oxygenation function, the $$S/F$$ S / F ratio. Because of the ceiling effect in oxyhaemoglobin saturation, $$S/F$$ S / F ratio ceases to reflect pulmonary oxygenation function at high $${S}_{p}{O}_{2}$$ S p O 2 values. We found that the correlation of $$S/F$$ S / F with the reference standard ( $${P}_{a}{O}_{2}$$ P a O 2 / $${F}_{I}{O}_{2}$$ F I O 2 ratio) improves substantially when excluding $${S}_{p}{O}_{2} > 0.94$$ S p O 2 > 0.94 and refer to this measure as $$S/{F}_{94}$$ S / F 94 . Using observational data from 39,765 hospitalised COVID-19 patients, we demonstrate that $$S/{F}_{94}$$ S / F 94 is predictive of mortality, and compare the sample sizes required for trials using four different outcome measures. We show that a significant difference in outcome could be detected with the smallest sample size using $$S/{F}_{94}$$ S / F 94 . We demonstrate that $$S/{F}_{94}$$ S / F 94 is an effective intermediate outcome measure in COVID-19. It is a non-invasive measurement, representative of disease severity and provides greater statistical power.