Background: Sweat conductivity (SC) is a semi-automated method widely used as a screening test for Cystic Fibrosis. Quantity not sufficient (QNS) is defined when collecting a volume lower than 15 μl of sweat during 30 minutes. Objective: To verify the rate and factors related to QNS for SC in newborns and young infants. Methods: Newborns and infants aged less than three months that had undergone sweat conductivity after two abnormal immunoreactive trypsinogen results, were recruited prospectively and consecutively. Statistical analysis included descriptive statistics, univariate and multivariate logistic regression. Results: A total of 1020 individuals were included. Among them, the rate of QNS was 8.9%. Subjects with gestational age <37 weeks (OR=5.0), birth weight <2.000g (OR=3.5), and daily weight gain <25g/day (OR=3.4) were more likely to produce an insufficient quantity of sweat. Conclusion: Our results suggest that QNS rates for SC could successfully fulfill the Cystic Fibrosis Foundation standards in newborns and young infants. In cases of QNS, SC should be scheduled as early as possible when the infant is older than 37 weeks (corrected age).
To the Editor, Sweat conductivity (SC) is a well-established screening test for Cystic Fibrosis (CF).1,2 It is semi-automated, fast, doesn’t require skilled technicians or specialized personnel, and needs a small amount of sweat. Moreover, one response to the demanding and labour-intensive requirement for robust quality sweat testing has been the testing of sweat samples by electrical conductivity.3As every laboratory method, SC is subject to error. Then, it should be assessed and tested to ensure that it produces results which make it reliable and suitable for the intended purpose, i.e., its role as screening method. Repeatability is one of these vital requirements.The International Organization for Standardization defines repeatability as the nearest agreement between independent test results performed in identical conditions in a short period of time, i.e., same test items and steps, same equipment, same technician, same laboratory.4Although SC is adopted worldwide there is no study evaluating specifically that requirement in any age group, including newborns and young infants. Therefore, the present study aimed to assess the correlation between two concomitant SC assays.We prospectively and consecutively recruited clinically stable infants younger than 3 months. They had two previous positive immunoreactive trypsin results (IRT), and then, two concomitant SC assays, performed by the same technician in the same facility. IRT and SC were carried out in the single accredited Reference Center for Newborn Screening and Genetic Diagnosis, located in the city of Belo Horizonte, State of Minas Gerais, Brazil.Sweat samples were obtaining from each forearm through the Wescor Macroduct system and then analyzed through a SC analyzer (Sweat-Chek analyzer, model 3120, Wescor Inc., USA). All steps were performed according to the manufacturer’s recommendations, and described elsewhere.1,2Apart from descriptive statistics, to assess the relationship between the two SC assays we used Spearman’s rank correlation coefficient. SPSS software, version 18.0 (SPSS Inc., Chicago, Illinois) was used for statistical analyses. The research protocol was approved by the Research Ethics Committee of Federal University of Minas Gerais, under number CAAE 21958014.1.0000.5149.A total of 322 young infants were recruited. Most of them (89%) were from smaller towns within the same State, and 11% lived in Belo Horizonte, the State capital. There was a slightly predominance of males (54%), and the majority (59%) were younger than 60 days, being 15.2% of them aged up to 30 days of life. Mean and median age was 52 and 46 days (range, 19-89 days), respectively.A sufficient amount of sweat was obtained from all participants. Means and medians for the first and second SC values were 37.0 mmol/L, and 34.3 mmol/L (range, 29.0-39.6 mmol/L), and 38.0 mmol/L and 35.3 mmol/L (range 30.0-40.7), respectively.Figure 1 displays the Spearman’s correlation scatter plot for the two SC results.