The present study shows that an abnormal N2-slope in combination with a FEV1 result < 90% of the basal value, predicts the development of BOS with a probability of 75%. Using either FEV1 or the N2-slope by themselves were not as useful. The single-breath N2-slope identified BOS and BOS 0-p in almost all cases, but since there were abnormal values among patients without BOS and BOS 0-p the positive predictive values were lower.
The BOS 0-p stage, i.e. an average decline in FEV1 of 10-19% of the basal value of two measurements at least 3 weeks apart, was added in 2002 to the BOS scoring system in an attempt of early detection of bronchiolitis obliterans [7]. Hachem et al reported a positive predictive value of BOS 0-p of 79% based on an analysis of 183 bilateral recipients including 72 BOS 0-p patients [8]. Our results indicate that the positive predictive value of diagnosed BOS 0-p is only 57%, but this value is based on a lower patient number and on the assumption that three patients who were diagnosed as BOS 0-p at the very last control visit did not develop BOS. If only one of these three BOS 0-p patients in fact did develop BOS, the positive predictive value would have been 71%.
Measurements of ventilation distribution by single-breath washouts for N2, helium, and sulfur hexafluoride have been shown to be more sensitive than conventional ventilation measures in the detection of ventilation disturbances in the peripheral airways in smokers, asthma, and cystic fibrosis [16, 20, 21]. Especially the slope of the single-breath N2 test proved highly sensitive in the detection of early disturbances of ventilation in the peripheral airways caused by tobacco smoke [16]. In that study, based on 97.5% specificity, 58% of 50 years old male heavy smokers (> 15 g/d) had abnormally steep N2 slope whereas only 28% had abnormal FEV1. Thus, there is comprehensive amount of evidence showing that single breath wash out tests are useful in early detection of abnormalities in peripheral airways.
A recent study by Van Muylem et al [22] showed that the helium slope of the single-breath washout was more sensitive in detecting BOS stage 0-p than exhaled biomarkers, i.e. NO and CO. Other authors have analysed the usefulness of the single-breath N2 slope to diagnose BOS. Reynaud-Gaubert et al [11] used the single-breath N2 slope similarly as in the present study and reported a sensitivity of 100% in the detection of BOS in 47 patients after heart-lung or double lung transplantation. In that study, however, the chosen cut off value of 3% N2/L obtained at least twice, was acquired in 6 out of 25 patients without BOS resulting in a positive predictive value of 78.5%. Estenne et al measured the slope of the alveolar plateau of various gases following inhalation of 1 L of a gas mixture of helium and sulfur hexaflouride and also found a very high sensitivity. The associated cut off values were defined by the confidence interval obtained in 10 stable lung transplant recipients, resulting in a specificity of 82% [12].
As any sensitivity may be achieved by choosing various cut off values, it is desirable that similar specificities are applied to facilitate comparisons. Regarding lung function tests it is generally accepted that the lower limit normal is defined as to result in 95% specificity. Accordingly the cut off value in the present study was chosen so that 95% of all measurements from 6 months post transplantation onward of the 43 stable patients were defined as normal. There appears to be no doubt in that indexes of ventilation distribution are in excellent agreement with the BOS diagnosis, i e having high sensitivitiy, but that abnormal values occur to some extent also among those patients who do not have BOS, resulting in less favorable positive predictive values.
The ability of various indexes of ventilation distribution to predict BOS was indicated in an earlier report [9] and has since been confirmed by others [11, 12] reporting the time delay between reaching the cut off and the diagnose of BOS. Estenne et al reported that an increased alveolar slope preceded BOS in 17 of 18 BOS patients, but this was also obtained in 7 of 39 patients who did not develop BOS within the course of the study [12]. Thus the positive predictive value was 71%. The present study shows that the probability of a single test result of the N2 slope to precede BOS within the three years follow up is only 56%, but when combined with concomitant FEV1 < 90% of the basal value, the probability increases to 75%, a value that may be considered clinically useful. A drawback is the relatively low prevalence of this combination. Considering the moderate regular control scheme in our transplantation unit compared to previous reports, the analysis based on single test results appears suitable but will necessarily result in more false positive results. Furthermore, as the number of BOS cases was rather small in the present study as well as in previous similar studies, the calculated characteristics are based on small numbers. Another limitation to our study is the retrospective study design which is a risk that the results can be more observational. Preferably, one should apply the N2-test in a larger patient cohort and in a prospective manner to ascertain how it would serve as an early predictor of BOS.
The data available for the study was a retrospective analysis of patients from 1990 up until 2003. The intention was to have a stable immunosuppressive protocol as well as CMV prophylaxis regimen for the studied cohort and minimize for possible confounding factors. Since 2003 the regime and prophylaxis have been altered according to new international guidelines.
The pulmonary function laboratory at our institution has considerable experience of the N2 test and all technicians performing the tests were experienced and used to the method. All measurements were performed blinded to the status of the patient. Furthermore, the applied algorithm for the determination of the slope is robust [16], i.e. the difference of N2 concentration between the closing point and TLC - 0,825 mL BTPS divided by the corresponding volume. The chosen cut off values depend on this algorithm and also on the applied reference equation [19] since the cut off values are expressed in percent of predicted normal.
A diagnosis of BOS depends on exclusion of other possible causes of a decline in FEV1[23]. In the present material, the numerous routinely performed clinical examinations, chest radiographs and surveillance bronchoscopies with BAL and TBB ensure that possible confounding factors are reasonably controlled for. The NoBOS patients, however, defined as not having BOS at the follow-up examinations, probably contain some patients who later will develop BOS. This circumstance may certainly influence the analysis, but presumably does not influence the comparison of the N2-slope and BOS 0-p regarding the predictability.
The treatment of BOS has over the years been unsuccessful, but more recently several promising therapeutic approaches have been proposed [24–26]. An intensified treatment at an early stage of the disease, i.e. BOS 0-p might prevent the irreversible functional impairment which otherwise would occur.