Background: Predictive equations have been proposedas a simpler alternative to hypoxic challengetesting (HCT) for determining the risk of in-flighthypoxia.Aim: To assess agreement between hypoxic challengetesting (HCT) and predictive equations forassessment of in-flight hypoxia.Design: Retrospective study.Methods: Patients with chronic obstructive pulmonarydisease (COPD) (n¼15), interstitial lung disease(ILD) (n¼15) and cystic fibrosis (CF) (n¼15) werestudied. Spirometry was recorded prior to hypoxicinhalation and oxygen saturations (SpO2) wererecorded before, after and during hypoxic inhalation.Blood gases were analysed before and afterhypoxic inhalation and when SpO2¼85%. An HCTwas performed using the Ventimask method.The PaO2 at altitude was estimated for each groupusing four published predictive equations, whichuse values of PaO2 (ground) and lung functionmeasurements to predict altitude PaO2. Results wereinterpreted using the BTS recommendations forprescription of in-flight oxygen post HCT. TheStuart Maxwell test of overall homogeneity wasused to assess agreement between HCT results andeach of the predictive equations.Results: Ground PaO2 was significantly greater inpatients with CF than either ILD or COPD (p <0.05).PaO2 in all three groups significantly decreasedfollowing HCT. With the exception of equation 3,significantly fewer patients in each group wouldrequire in-flight O2 if prescription was based onHCT, compared to predictive equations (p <0.05).Discussion: Predictive equations considerablyoverestimate the need for in-flight O2, comparedto HCT.