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.3
As 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.4
Although 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.1,2
Apart 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.