Background and Aims TLNO, transfer factor for nitric oxide, is a pulmonary function test of gas transfer. The test, in combination with carbon monoxide, allows for calculation of the alveolar membrane diffusing capacity and red blood cell conductance. This allows physicians to recognise physiologically where issues with gas transfer arise. Currently, there are very few papers looking at TLNO reproducibility in healthy subjects and none aiming with a target of 5% repeatability between efforts. This study attempts to look at the reproducibility of TLNO over 10 sessions (7 weeks total) with an intra-session repeatability of 5%. In addition, comparison of TLCO measurements between 10 and 5 s breath holds are made.

Methods 14 normal subjects were recruited and a baseline spirometry was taken and height, weight, age and sex were recorded. Subjects were asked to perform a TLCO test with 10 s breath hold followed by 10 repeated sessions of the TLNO test on different days. Measurements within 5% of each other were considered acceptable repeated Results in one session. A Bland-Altman plot and regression line were constructed to compare TLCO measures between different breath hold times. One-way repeated measures ANOVA, measurement error values, intra and inter-session variability were calculated for TLNO and TLCO recordings obtained over the 10 repeated sessions.

Results Bland-Altman plot revealed no statistically significant (p=0.783, p≥0.05) difference between TLCO breath hold times. Coefficient of determination from the regression line, r²=0.860. Repeated measures ANOVA revealed no significant difference for TLNO and TLCO measurements over time at p=0.374 and p=0.842 (p≥0.05) respectively. Intra-session and inter-session variability for TLNO were calculated as 15.02 ml/min/mmHg and 16.12 ml/min/mmHg respectively. TLCO intra-session and inter-session variability were 4.30 ml/min/mmHg and 3.70 ml/min/mmHg. We have shown that TLCO values recorded with the shorter 5 s breath hold agree with the conventional 10 s technique. Over a 7 week period TLNO and TLCO do not change significantly and calculated session variability is consistent with ERS/ATS guidelines.

Conclusion TLNO is a highly reproducible test over a 7 week period and a shortened breath hold in healthy people provides the same values as the traditional 10 s breath hold for TLCO.