The most common preanalytic problem of sweat testing: Insufficient sweat volume C ystic fibrosis (CF) is a fatal disease, an inherited autosomal recessive disorder resulting from mutations in the CF transmembrane conductance regulator gene, located in the long arm of chromosome 7 [1]. The prevalence is about 1 in 3500 live births [2]. According to the limited number of studies, the incidence of CF is about 1/3000 in Turkey. However, the rate is thought to be higher due to the frequency of consanguineous marriage [3]. In studies, early diagnosis and treatment has led to improved nutritional status and lung function, prolonging the life span by reducing hospitalizations and morbidity [1, 4]. Therefore, CF has been part of newborn screening programs in some parts of the world for many years [5]. In Turkey, all newborns have been screened for immunoreactive trypsinogen from a dried blood spot since January 1, 2015 and positive cases are referred for a sweat test. The sweat test is the gold standard method for the diagnosis of CF based on the quantitative determination of electrolytes in the sweat sample [6]. The test consists of 3 steps: Stimulation of sweat glands by pilocarpine iontophoresis, sweat collection, and analysis. The primary sweat collection methods Objectives: A sweat test is the gold standard method for the diagnosis of cystic fibrosis (CF). The most important preanalytical error for sweat testing is insufficient sweat volume. The aim of this study was to determine rates of insufficient sweat volume and evaluate the relationship of sweat volume with demographic characteristics of patients, as well as the implementation of corrective preventive actions. Methods: This study was performed retrospectively in the sweat test laboratory of the Cerrahpasa Faculty of Medicine of Istanbul University-Cerrahpasa. A total of 545 specimens that were referred to the laboratory between May and December 2016 were evaluated. Sweating was stimulated with pilocarpine iontophoresis, and the Macroduct Coil System (Wescor, Inc., Logan, UT, USA) was used as the sweat collection system. Sweat volume <15 μL was evaluated as quantity not sufficient (QNS). A chi-square test was used for statistical analysis; p<0.05 was considered significant. Results: The QNS rate in the study laboratory was 13.8% for infants ≤3 months of age and 6% for patients aged >3 months (p=0.019). There was no significant difference in the QNS ratios according to gender (p=1.000) or the season when the test was applied (p=0.181). Conclusion: The determination of QNS rates is a crucial step in the standardization of the preanalytical conditions of a sweat test. In this study, the QNS rate was above that recommended by the Cystic Fibrosis Foundation (<10% for infants ≤3 months of age, <5% for patients >3 months). High QNS ratios may be due to the inadequacy of optimization of pre-test preparation. Laboratory staff and parents should be well informed about preanalytical factors before a sweat analysis is performed in order to reduce the QNS rate.