ABSTRACT
BACKGROUND AND OBJECTIVES: The diagnosis, assessment and management of a wide range of respiratory diseases rely on accurate interpretation of lung function tests through the use of reference equations to generate predicted values. This paper ascertains the suitability of reference equations currently used in New Zealand through comparison with newly derived equations from the Wellington Respiratory Survey, and discusses the relevance of the findings to the Asia Pacific region. METHODS: A survey of lung function testing facilities determined the reference equations in common usage. Pulmonary function test results from healthy, lifelong non-smoking subjects (n = 180) were expressed as percentage predicted values, with comparisons made between the currently used and Wellington Respiratory Survey reference equations. Differences in disease severity classification in subjects with COPD (n = 46) and asthma (n = 61) were determined, using the different reference equations. RESULTS: Currently used equations significantly underpredict measured values for FEV(1), PEF, TLC and RV by up to 20%. Severity classification of COPD and asthma based on per cent predicted FEV(1) was substantially altered by the choice of reference equation. CONCLUSION: Many reference equations in current usage in New Zealand are no longer suitable for use. The applicability of reference equations used in other populations and countries within the Asia Pacific region requires further investigation. We recommend that up-to-date reference equations are derived and implemented if those currently used are shown to be unsatisfactory.
Subject(s)
Models, Biological , Respiratory Function Tests/methods , Adult , Aged , Female , Health Surveys , Humans , Lung/physiology , Lung/physiopathology , Lung Diseases/classification , Lung Diseases/physiopathology , Male , Middle Aged , New Zealand , Predictive Value of Tests , Reference Values , Severity of Illness Index , SpirometryABSTRACT
BACKGROUND: The role of computerised tomography (CT) lung density measurements in objective quantification of emphysema is uncertain. The aim of this study was to determine normal reference values for CT lung density measurements and investigate their utility in identifying subjects with clinical emphysema. METHODS: Normal subjects (non-smokers, no respiratory disease, n=185) and subjects with clinical emphysema (post-bronchodilator FEV(1)/FVC <70%, > or =10 pack years tobacco smoking, no childhood asthma and, either D(LCO)/VA <80% predicted and/or macroscopic emphysema on CT, n=22) were identified from a random population survey. Subjects underwent CT scanning, with measurement of areas of low attenuation as a percentage of total area (RA%) for three standardised slices and two reconstruction algorithms with a density threshold of -950 HU. Reference values in normal subjects, and ability of the measurements to discriminate between the two groups were determined. RESULTS: Reference values for individual subjects showed wide confidence intervals (standard resolution scans, RA% females 0.2-3.9%, males 0.4-8.7%.) Subjects with emphysema had greater RA% values compared with normal subjects, the difference being most marked in apical slices (standard resolution algorithm, apical slice, median RA% 2.9% (95% CI 0.4-11.1%) vs. 0.1% (95% CI 0.0-0.5%), emphysema vs. normal subjects, respectively). Logistic regression analysis showed poor discriminant ability to distinguish between the groups, the most favourable cut-off yielding a sensitivity and specificity of 83.3% and 62.8%, respectively. CONCLUSIONS: CT lung density measurements cannot reliably detect the presence of emphysema in an individual. We recommend further investigation into lung density measurements before their widespread use in clinical practice.
Subject(s)
Lung/diagnostic imaging , Pulmonary Emphysema/diagnostic imaging , Adult , Aged , Aging/physiology , Algorithms , Female , Humans , Image Processing, Computer-Assisted/methods , Male , Middle Aged , Reference Values , Respiratory Function Tests , Sex Characteristics , Tomography, X-Ray Computed/methodsABSTRACT
BACKGROUND: Objective quantification of emphysema using computerized tomography (CT) density measurements is rapidly gaining wide acceptance as an in vivo measurement tool. However, some studies have suggested that abnormal lung function in the absence of emphysema can affect lung density, and the role of such measurements in identifying and monitoring the progression of emphysema is not clear. OBJECTIVE: To clarify the relationship between lung density measurements and pulmonary function. METHODS: CT measurements of the proportion of lung occupied by low density tissue (as percentage of lung area below predetermined Hounsfield unit [HU] thresholds) were obtained in a large random population (n = 739) and the association with detailed pulmonary function tests studied using factor analysis. RESULTS: Density measurements showed a greater association with measures of hyperinflation and airflow obstruction than measures of gas transfer (correlation coefficient, high resolution scan, -950HU threshold vs FEV1/FVC, RV, and DLCO/VA of -0.39, 0.22, and -0.15 respectively). The strongest lung density factor coefficients of 0.51 (standard resolution scan, - 950 HU threshold) and 0.46 (high resolution scan, - 910 HU threshold) were seen with factors predominantly consisting of measures of airflow obstruction and hyperinflation. Most variation in lung density was not accounted for by lung function measurements (communality 0.21-0.34). CONCLUSION: Lung density measurements associate most strongly with measures of airway disease that are not specific to emphysema.
