Spirometer + body box = VW beetle + Mercedes?

Diagnostic accuracy of FeNO [fractional exhaled nitric oxide] and asthma symptoms increased when evaluated with a superior reference standard

OBJECTIVES

The objective of the study is to determine the impact of changing reference standards (RS), namely spirometry vs. whole-body plethysmography (WBP), on estimation of the diagnostic accuracy of fractional exhaled nitric oxide (FeNO) and clinical signs and symptoms (CSS) as index tests regarding asthma diagnosis.

STUDY DESIGN AND SETTING

This was a diagnostic study conducted in 393 patients attending a private practice of pneumologists with complaints suspicious of asthma. First, the index tests were compared with the diagnostic results of spirometry in terms of forced expiratory volume in the first second (FEV₁) responsiveness. Second, the index tests were compared with the results of WBP in terms of specific airway resistance and FEV₁ responsiveness. Areas under the curve (AUC) were compared with a generalized estimating equation approach based on binary logistic regression.

RESULTS

FeNO values and CSS 'wheezing' and 'allergic rhinitis' showed higher specificities (P < 0.001) and sensitivities (not significant) when evaluated with WBP; also, Youden indices increased in these CSS (P < 0.05). AUC of FeNO in combination with 'wheezing' and 'allergic rhinitis' when WBP was used as RS (AUC = 0.724; 95% confidence interval 0.672 to 0.776) was higher compared with spirometry as RS (AUC = 0.654; 95% confidence interval 0.585 to 0.722) (P < 0.001).

CONCLUSION

In case of asthma, superior RS led to more favorable assessment of index tests. FeNO measurement might have been underestimated in some previous studies.

Whole-Body Plethysmography in Suspected Asthma: A Prospective Study of Its Added Diagnostic Value in 302 Patients

BACKGROUND

Whole-body plethysmography (WBP) with bronchial challenge testing to measure the (specific) airway resistance, (s)R(AW), is considered to be a more sensitive diagnostic procedure than spirometry, which can only measure the forced expiratory volume in one second (FEV1). The evidence for the added diagnostic value of WBP is not yet conclusive.

METHODS

In a prospective diagnostic study, we carried out WBP with bronchial challenge testing as well as a bronchodilation test in 400 patients with suspected asthma from June 2010 to October 2011. The bronchial provocation test was considered positive if the FEV1 fell by at least 20% and/or the airway resistance doubled, with an increase of the sR(AW) to at least 2.0 kPA × s and/or of the R(AW) to 0.5 kPA × s/L. Follow-up evaluation was performed one year later.

RESULTS

The prevalence of asthma in the 302 patients who completed follow-up was 27.5%. The sensitivity of WBP with sR(AW) measurement for asthma was 95.2% (95% confidence interval [CI] 88.3%-98.1%), and its specificity was 81.7% (95% CI 76.1%-86.3%). The sensitivity of FEV1 was 44.6% (95% CI 34.4%-55.3%), and its specificity was 91.3% (95% CI 86.6%-94.4%). The negative predictive value (NPV) of WBP with sR(AW) measurement was 97.8% (95% CI 94.5%-99.1%), while that of FEV1 was 81.3% (95% CI 76.0%-85.7%). The positive predictive value (PPV) of WBP with sR(AW) measurement was 66.4% (95% CI 57.5%-74.2%), while that of FEV1 was 66.1% (95% CI 53.0%-77.1%).

CONCLUSION

With sR(AW) measurement, asthma can be ruled out with high certainty. Improving the positive predictive value of testing for asthma remains a challenge, however, as sR(AW) measurement does not yield any increase in specificity.