Analytical and biological variation in repeated sweat chloride concentrations in clinical trials for CFTR modulator therapy

BACKGROUND

Using sweat chloride as a biomarker for CFTR modifying drugs requires knowledge of analytical and biological variation.

METHODS

979 sweat chloride concentrations from 128 subjects enrolled in the placebo arm of 2 multicenter, investigational drug trials were analyzed to determine coefficients of variation (CV) as well as reference change value (RCV) and index of individuality (II).

RESULTS

For these populations, calculated values for the two studies were: analytical variation (3.9, 4.1%); within-subject variation (4.4, 6.0%); between-subject variation (8.9, 7.0%); RCV (13.7, 17.0%) and II (0.7, 1.0). Sweat chloride variation was not affected by sex, collection site or sample weight; but was slightly affected by age in one of the two studies.

CONCLUSION

Through determination of analytical as well as between- and within-subject variation, and with a larger sample size, our data allows improved estimates of the RCV and II, and can contribute to future trials of CFTR modulators and inform the design and interpretation of n of 1 trials in both research and clinical settings.

Performance Evaluation of a New Coulometric Endpoint Method in Sweat Testing and Its Comparison With Classic Gibson&Cooke and Chloridometer Methods in Cystic Fibrosis

Background: The objective of the study was to assess the diagnostic efficacy of the coulometric endpoint method and compare it with classic Gibson&Cooke and chloridometer methods. Methods: This study is a prospective clinical study comparing two conventional sweat testing methods with the coulometric endpoint method in previously diagnosed cystic fibrosis (CF) patients and a non-CF control group. All individuals underwent two simultaneous sweat collections. One sample of sweat, collected by the CFΔ collector coil system, was analyzed by two methods: the titrimetric Cl^- measurement (Sherwood® Chloridometer 926S, Sherwood Scientific Ltd., Cambridge, UK) and the coulometric endpoint method (CF Δ Collection System®, UTSAT/Turkey); the second sample was collected from the other forearm by the Gibson&Cooke method and the collected sweat was analyzed by manual titration in accordance with the Schales&Schales method. Within-run and between-run imprecisions were evaluated via Cl^- concentrations of 40, 70, and 130 mmol/L samples. Results: One hundred and seventy (60 CF and 110 controls) subjects were included in the study. All three sweat test methods discriminated CF subjects from the healthy individuals. The mean difference between the coulometric endpoint and titrimetric Cl^- measurement methods was -1.5 mmol/L, (95% confidence limits of agreement, ranging from -8.9 to 15.9 mmol/L); the mean difference between manual titration vs. coulometric endpoint methods was 12.8 mmol/L, (95% confidence limits of agreement ranging from -9.7 to 45.3 mmol/L) and the mean difference between the manual titration and titrimetric Cl^- measurement methods was 11.3 mmol/L, (95% confidence limits of agreement ranging from -7.8 to 40.5 mmol/L) based on a Bland-Altman analysis. In the Receiver operating characteristic (ROC) analysis, made on the basis that Cl^- concentration values < 40 mmol/L exclude the CF diagnosis, the coulometric endpoint method resulted in 96.7% sensitivity and 100% specificity for a cut-off value of 58.5 mmol/L (AUC: 0.994; 95% CI = 0.986-1.000; p < 0.001). Conclusions: The coulometric endpoint method can be as reliable as quantitative sweat Cl^- analysis and may be considered as a definitive diagnostic tool for CF.

Biological Variation of Chloride and Sodium in Sweat Obtained by Pilocarpine Iontophoresis in Adults: How Sure are You About Sweat Test Results?

INTRODUCTION

The measurement of chloride and sodium concentrations in sweat is an important test for the diagnosis of cystic fibrosis (CF). The aim of this study was to assess the analytical variation (CV_A) and within-subject (CV_I) and between-subject (CV_G) biological variation of chloride and sodium concentrations in sweat, collected by pilocarpine iontophoresis and to determine their effect on the clinical interpretation of sweat test results.

METHODS

Twelve Caucasian adults (six male and six female) without symptoms suggestive for CF and with a mean age of 41 years (range 28-59) were included in the study. At least eight samples of sweat were collected from each individual by pilocarpine iontophoresis. Chloride and sodium concentrations were measured in duplicate for each sample using ion selective electrodes. After the removal of outliers, the CV_A, CV_I, and CV_G of chloride and sodium were determined, and their impact on measurement uncertainty and reference change value were calculated.

RESULTS

The CV_A, CV_I, and CV_G of chloride in sweat samples were 6.5, 17.7, and 47.2%, respectively. The CV_A, CV_I, and CV_G of sodium sweat samples were 6.0, 17.5, and 42.6%, respectively.

CONCLUSION

Our study indicates that sweat chloride and sodium concentration results must be interpreted with great care. Different components of variation, particularly the biological variations, have a considerable impact on the interpretation of these results. If no pre-analytical, analytical, or post-analytical errors are suspected, repeated sweat testing to confirm first-measurement results might not be desirable.

Sources of Variation in Sweat Chloride Measurements in Cystic Fibrosis

RATIONALE

Expanding the use of cystic fibrosis transmembrane conductance regulator (CFTR) potentiators and correctors for the treatment of cystic fibrosis (CF) requires precise and accurate biomarkers. Sweat chloride concentration provides an in vivo assessment of CFTR function, but it is unknown the degree to which CFTR mutations account for sweat chloride variation.

OBJECTIVES

To estimate potential sources of variation for sweat chloride measurements, including demographic factors, testing variability, recording biases, and CFTR genotype itself.

METHODS

A total of 2,639 sweat chloride measurements were obtained in 1,761 twins/siblings from the CF Twin-Sibling Study, French CF Modifier Gene Study, and Canadian Consortium for Genetic Studies. Variance component estimation was performed by nested mixed modeling.

MEASUREMENTS AND MAIN RESULTS

Across the tested CF population as a whole, CFTR gene mutations were found to be the primary determinant of sweat chloride variability (56.1% of variation) with contributions from variation over time (e.g., factors related to testing on different days; 13.8%), environmental factors (e.g., climate, family diet; 13.5%), other residual factors (e.g., test variability; 9.9%), and unique individual factors (e.g., modifier genes, unique exposures; 6.8%) (likelihood ratio test, P < 0.001). Twin analysis suggested that modifier genes did not play a significant role because the heritability estimate was negligible (H2 = 0; 95% confidence interval, 0.0-0.35). For an individual with CF, variation in sweat chloride was primarily caused by variation over time (58.1%) with the remainder attributable to residual/random factors (41.9%).

CONCLUSIONS

Variation in the CFTR gene is the predominant cause of sweat chloride variation; most of the non-CFTR variation is caused by testing variability and unique environmental factors. If test precision and accuracy can be improved, sweat chloride measurement could be a valuable biomarker for assessing response to therapies directed at mutant CFTR.

Sixty-five years since the New York heat wave: advances in sweat testing for cystic fibrosis

The sweat test remains important as a diagnostic test for cystic fibrosis (CF) and has contributed greatly to our understanding of CF as a disease of epithelial electrolyte transport. The standardization of the sweat test, by Gibson and Cooke [Gibson and Cooke (1959) Pediatrics 1959;23:5], followed observations of excessive dehydration amongst patients with CF and confirmed the utility as a diagnostic test. Quantitative pilocarpine iontophoresis remains the gold standard for sweat induction, but there are a number of collection and analytical methods. The pathophysiology of electrolyte transport in sweat was described by Quinton [Quinton (1983) Nature 1983;301:421-422], and this complemented the developments in genetics that discovered the cystic fibrosis transmembrane conductance regulator (CFTR), an epithelial-based electrolyte transport protein. Knowledge of CF has since increased rapidly and further developments in sweat testing include: new collection methods, further standardization of the technique with international recommendations and age related reference intervals. More recently, sweat chloride values have been used as proof of effect for the new drugs that activate CFTR. However, there remain issues with adherence to sweat test guidelines in many countries and there are gaps in our knowledge, including reference intervals for some age groups and stability of sweat samples in transport. Furthermore, modern methods of elemental quantification need to be explored as alternatives to the original analytical methods for sweat electrolyte measurement. The purpose of this review is therefore to describe the development of the sweat test and consider future directions.

Recommendations for the classification of diseases as CFTR-related disorders

Several diseases have been clinically or genetically related to cystic fibrosis (CF), but a consensus definition is lacking. Here, we present a proposal for consensus guidelines on cystic fibrosis transmembrane conductance regulator (CFTR)-related disorders (CFTR-RDs), reached after expert discussion and two dedicated workshops. A CFTR-RD may be defined as "a clinical entity associated with CFTR dysfunction that does not fulfil diagnostic criteria for CF". The utility of sweat testing, mutation analysis, nasal potential difference, and/or intestinal current measurement for the differential diagnosis of CF and CFTR-RD is discussed. Algorithms which use genetic and functional diagnostic tests to distinguish CF and CFTR-RDs are presented. According to present knowledge, congenital bilateral absence of vas deferens (CBAVD), acute recurrent or chronic pancreatitis and disseminated bronchiectasis, all with CFTR dysfunction, are CFTR-RDs.