Sweat Chloride Concentration Research Articles

In vivo and in vitro ivacaftor response in cystic fibrosis patients with residual CFTR function: N-of-1 studies.

Ivacaftor, a cystic fibrosis transmembrane conductance regulator (CFTR) potentiator, decreases sweat chloride concentration, and improves pulmonary function in 6% of cystic fibrosis (CF) patients with specific CFTR mutations. Ivacaftor increases chloride transport in many other CFTR mutations in non-human cells, if CFTR is in the epithelium. Some CF patients have CFTR in the epithelium with residual CFTR function. The effect of ivacaftor in these patients is unknown. This was a series of randomized, crossover N-of-1 trials of ivacaftor and placebo in CF patients ≥8 years old with potential residual CFTR function (intermediate sweat chloride concentration, pancreatic sufficient, or mild bronchiectasis on chest CT). Human nasal epithelium (HNE) was obtained via nasal brushing and cultured. Sweat chloride concentration change was the in vivo outcome. Chloride current change in HNE cultures with ivacaftor was the in vitro outcome. Three subjects had decreased sweat chloride concentration (-14.8 to -40.8 mmol/L, P < 0.01). Two subjects had unchanged sweat chloride concentration. Two subjects had increased sweat chloride concentration (+23.8 and +27.3 mmol/L, P < 0.001); both were heterozygous for A455E and pancreatic sufficient. Only subjects with decreased sweat chloride concentration had increased chloride current in HNE cultures. Some CF patients with residual CFTR function have decreased sweat chloride concentration with ivacaftor. Increased chloride current in HNE cultures among subjects with decreased sweat chloride concentrations may predict clinical response to ivacaftor. Ivacaftor can increase sweat chloride concentration in certain mutations with unclear clinical effect. Pediatr Pulmonol. 2017;52:472-479. © 2017 Wiley Periodicals, Inc.

PharmGKB summary: ivacaftor pathway, pharmacokinetics/pharmacodynamics.

Ivacaftor [VX-770; N-(2,4-Di-tert-butyl-5-hydroxyphenyl)-4-oxo-1,4-dihydroquinoline-3-carboxamide] is one of the first drugs developed to treat an underlying cause of cystic fibrosis (CF) rather than the symptoms. It is a selective small molecule potentiator of the Cystic Fibrosis Transmembrane Conductance Regulator protein (CFTR). Potentiators are a new drug class of CFTR modulators designed to restore CFTR function [1]. The Food and Drug Administration (FDA) has granted ivacaftor orphan drug status because it is indicated in a subset of patients with particular CFTR variants, specifically class III variants. Approximately 30,000 people have CF in the United States, which is below the 200,000 patient threshold for orphan drugs in the United States [1]. CF is an autosomal recessive disorder characterized by sweat chloride concentration above 60 mmol/L, leading to progressive obstructive lung disease and premature mortality, as well as problems in the liver, pancreas, vas deferens, and intestine [2]. CF affects approximately 70,000 people worldwide [3], including approximately 1 of every 3500 infants born in the United States [4], and is caused by inheriting two detrimental copies of the CFTR gene. The CFTR protein is a chloride channel, which maintains ion and water balance intra- and extra-cellularly [5]. The channel opens and closes through ATP binding, hydrolysis, and phosphorylation, which changes the protein conformation to allow chloride ions to flow through the ion gradient between the intracellular and extracellular regions [2]. Defective CFTR function prevents the flow of chloride ions across epithelial cells, which leads to the mucus buildup, infection, inflammation, and progressively decreasing lung function that is characteristic of cystic fibrosis [5].

Lung function and disease severity in cystic fibrosis patients heterozygous for p.Arg117His.

Expression of p.Arg117His cystic fibrosis (CF) transmembrane conductance regulator is influenced by a polythymidine (poly-T) tract and a thymidine–guanine (TG) repeat on intron 9, which vary in length and affect exon 10 skipping.We compared clinical characteristics and the rate of progression of lung disease of CF patients carrying the p.Arg117His mutation with different intron 9 varying sequences (poly-T) and mutation classes in trans.Data were collected from patients in Northern Ireland, UK, including diagnostic features, sweat chloride, nutritional status, sputum microbiology, CF-related complications and lung function. Poly-T and TG repeats were determined by PCR. Forced expiratory volume in 1 s (FEV1) decline was determined from linear regression of FEV1 measurements of patients over time.We identified 62 patients with p.Arg117His, 55 with a class I/II mutation in trans and six with p.Arg117His/p.Gly551Asp. 42 patients had 5T and 13 had 7T. All patients had 12 TG repeats. Patients with p.Arg117His-5T had greater lung function decline, sweat chloride concentrations, pancreatic insufficiency and prevalence of Pseudomonas aeruginosa infection compared with patients with p.Arg117His-7T.Lung function decline and disease severity in p.Arg117His is determined by the poly-T tract length and identity of the mutation in trans. Patients with p.Arg117His-5T and a second class I/II mutation have a severity similar to p.Phe508del homozygous patients, although lung function decline is delayed to an older age. There may be linkage disequilibrium between p.Arg117His and 12 TG repeats.

Sources of Variation in Sweat Chloride Measurements in Cystic Fibrosis.

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

Deep resequencing of CFTR in 762 F508del homozygotes reveals clusters of non-coding variants associated with cystic fibrosis disease traits.

Extensive phenotypic variability is commonly observed in individuals with Mendelian disorders, even among those with identical genotypes in the disease-causing gene. To determine whether variants within and surrounding CFTR contribute to phenotypic variability in cystic fibrosis (CF), we performed deep sequencing of CFTR in 762 patients homozygous for the common CF-causing variant, F508del. In phase 1, ~200 kb encompassing CFTR and extending 10 kb 5′ and 5 kb 3′ of the gene was sequenced in 486 F508del homozygotes selected from the extremes of sweat chloride concentration. In phase 2, a 510 kb region, which included the entire topologically associated domain of CFTR, was sequenced in 276 F508del homozygotes drawn from extremes of lung function. An additional 163 individuals who carried F508del and a different CF-causing variant were sequenced to inform haplotype construction. Region-based burden testing of both common and rare variants revealed seven regions of significance (α=0.01), five of which overlapped known regulatory elements or chromatin interactions. Notably, the −80 kb locus known to interact with the CFTR promoter was associated with variation in both CF traits. Haplotype analysis revealed a single rare recombination event (1.9% frequency) in intron 15 of CFTR bearing the F508del variant. Otherwise, the majority of F508del chromosomes were markedly similar, consistent with a single origin of the F508del allele. Together, these high-resolution variant analyses of the CFTR locus suggest a role for non-coding regulatory motifs in trait variation among individuals carrying the common CF allele.

Intra-individual biological variation in sweat chloride concentrations

Intra-individual biological variation in sweat chloride concentrations

Cystic Fibrosis in An Infant: Presented as Pseudo Bartter Syndrome

Cystic Fibrosis (CF) can involve multiple organs although the most commonly affected systems are respiratory and gastrointestinal ones. In infancy it can also present as Pseudo Bartter Syndrome which is characterized by hyponatremic, hypochloremic, hypokalamic metabolic alkalosis without renal tubular pathology. We report a 5 month old boy who had history of recurrent episodes of dehydration due to vomiting and recurrent respiratory infection. His biochemical parameters suggestive of Pseudo Bartters Syndrome. Initial sweat chloride test was normal 16 mEq/L whereas the repeated test revealed Sweat chloride concentration of 107 mEq/L. The cystic fibrosis mutation analysis revealed F508 del homozygosity for the Cystic Fibrosis Transmembrane Regulator (CFTR) gene. Cystic fibrosis should be always considered in any infant with metabolic alkalosis and hyponatremic hypochloremic dehydration, whether or not there are association with pulmonary and /or gastrointestinal symptoms. Early diagnosis is essential in improving the prognosis and long term survival of these children.Chatt Shi Hosp Med Coll J; Vol.15 (1); Jan 2016; Page 57-59

23 Stratifying young children with cystic fibrosis for disease severity using intestinal organoid swelling, intestinal current measurements or sweat chloride concentration as CFTR-dependent biomarker

23 Stratifying young children with cystic fibrosis for disease severity using intestinal organoid swelling, intestinal current measurements or sweat chloride concentration as CFTR-dependent biomarker

Niejednoznaczna diagnoza mukowiscydozy u dzieci z pozytywnym wynikiem badania przesiewowego

IntroductionThe implementation of newborn screening for cystic fibrosis enabled early diagnosis and treatment of this disease. The diagnosis of cystic fibrosis occurs when there is a significantly elevated concentration of chloride in the sweat or CF-causing mutations are present in both alleles of the CFTR gene. CFSPID (Cystic Fibrosis Screen Positive Inconclusive Diagnosis) corresponds to a situation when CF cannot be either diagnosed or clearly ruled out. ObjectiveThe aim of this study was to present the characteristics of children with abnormal screening in whom it was impossible to recognize or rule out cystic fibrosis. MethodsThe study included patients with abnormal newborn screening test, performed by the Institute of Mother and Child from 01.09.2006 until 30.09.2014, born in four provinces, which were under our further care, and who met criteria of CFSPID. ResultsDuring the period of study, the diagnosis of cystic fibrosis was established in 142 patients. In 75 patients, not only was cystic fibrosis not diagnosed but also it could not be clearly ruled out. The number of such children has increased significantly after 2010. In the study group, the mean value of sweat chloride concentration was 26.6±9.6mmol/l. In every patient, two mutations in the CFTR gene were detected. Sixty-eight children (90%) had a cystic fibrosis causing mutation in one allele and a CFTR-related disorder causing mutation or the one with uncertain clinical consequences on the other allele. ConclusionsThe introduction of CF NBS has not only enabled early diagnosis of CF, but has also helped to detect changes in the CFTR with unclear clinical consequences. Changes are necessary, in order to reduce the detection of mutations of unclear phenotypic consequences.

Acute administration of ivacaftor to people with cystic fibrosis and a G551D-CFTR mutation reveals smooth muscle abnormalities.

Airflow obstruction is common in cystic fibrosis (CF), yet the underlying pathogenesis remains incompletely understood. People with CF often exhibit airway hyperresponsiveness, CF transmembrane conductance regulator (CFTR) is present in airway smooth muscle (ASM), and ASM from newborn CF pigs has increased contractile tone, suggesting that loss of CFTR causes a primary defect in ASM function. We hypothesized that restoring CFTR activity would decrease smooth muscle tone in people with CF. To increase or potentiate CFTR function, we administered ivacaftor to 12 adults with CF with the G551D-CFTR mutation; ivacaftor stimulates G551D-CFTR function. We studied people before and immediately after initiation of ivacaftor (48 hours) to minimize secondary consequences of CFTR restoration. We tested smooth muscle function by investigating spirometry, airway distensibility, and vascular tone. Ivacaftor rapidly restored CFTR function, indicated by reduced sweat chloride concentration. Airflow obstruction and air trapping also improved. Airway distensibility increased in airways less than 4.5 mm but not in larger-sized airways. To assess smooth muscle function in a tissue outside the lung, we measured vascular pulse wave velocity (PWV) and augmentation index, which both decreased following CFTR potentiation. Finally, change in distensibility of <4.5-mm airways correlated with changes in PWV. Acute CFTR potentiation provided a unique opportunity to investigate CFTR-dependent mechanisms of CF pathogenesis. The rapid effects of ivacaftor on airway distensibility and vascular tone suggest that CFTR dysfunction may directly cause increased smooth muscle tone in people with CF and that ivacaftor may relax smooth muscle. This work was funded in part from an unrestricted grant from the Vertex Investigator-Initiated Studies Program.

Variability of sweat chloride concentration in subjects with cystic fibrosis and G551D mutations

IntroductionSweat chloride concentration, a biomarker of CFTR function, is an appropriate outcome parameter in clinical trials aimed at correcting the basic CF defect. Although there is consensus on a cut-off value to diagnose CF, we have only limited information on the within subject variability of sweat chloride over time. Such information would be useful for sample size calculations in clinical trials. Therefore, we retrospectively analyzed repeated sweat chloride values obtained in patients with G551D mutation(s) assigned to placebo in an ivacaftor interventional trial. MethodsIn subjects with G551D at least 12years of age, a pilocarpine sweat test using Macroduct collector was taken on both arms at 8 time points over 48weeks. We explored 1062 pilocarpine sweat test values obtained in 78 placebo patients of the VX08-770-102 trial. ResultsMean overall sweat chloride value (all patients, all tests, n=1062) was 100.8mmol/L (SD 12.7mmol/L). Using a multilevel mixed model, the between-subject standard deviation (SD) for sweat chloride was 8.9mmol/L (95% CI 7.4–10.6) and within-subject SD was 8.1mmol/L (95% CI 7.5–8.7). Limits of repeatability for repeat measurements were −19.7 to +21.6mmol/L using values from one arm, and −13.3 to 11.8mmol/L using mean of values obtained at 4 test occasions. Sample size calculations showed that the minimal treatment effect on sweat chloride concentration that can be demonstrated for a group of 5 patients is around 15mmol/L, using a cross-over design and combinations of 4 tests for each phase of the trial. ConclusionAlthough the sweat test is considered a robust measure, sweat chloride measurements in patients with CF and a G551D mutation had an inherent biological variability that is higher than commonly considered. Further analyses of placebo group data are crucial to learn more about the natural variability of this outcome parameter.

Loss of carbonic anhydrase XII function in individuals with elevated sweat chloride concentration and pulmonary airway disease.

Elevated sweat chloride levels, failure to thrive (FTT), and lung disease are characteristic features of cystic fibrosis (CF, OMIM #219700). Here we describe variants in CA12 encoding carbonic anhydrase XII in two pedigrees exhibiting CF-like phenotypes. Exome sequencing of a white American adult diagnosed with CF due to elevated sweat chloride, recurrent hyponatremia, infantile FTT and lung disease identified deleterious variants in each CA12 gene: c.908-1 G>A in a splice acceptor and a novel frameshift insertion c.859_860insACCT. In an unrelated consanguineous Omani family, two children with elevated sweat chloride, infantile FTT, and recurrent hyponatremia were homozygous for a novel missense variant (p.His121Gln). Deleterious CFTR variants were absent in both pedigrees. CA XII protein was localized apically in human bronchiolar epithelia and basolaterally in the reabsorptive duct of human sweat glands. Respiratory epithelial cell RNA from the adult proband revealed only aberrant CA12 transcripts and in vitro analysis showed greatly reduced CA XII protein. Studies of ion transport across respiratory epithelial cells in vivo and in culture revealed intact CFTR-mediated chloride transport in the adult proband. CA XII protein bearing either p.His121Gln or a previously identified p.Glu143Lys missense variant localized to the basolateral membranes of polarized Madin-Darby canine kidney (MDCK) cells, but enzyme activity was severely diminished when assayed at physiologic concentrations of extracellular chloride. Our findings indicate that loss of CA XII function should be considered in individuals without CFTR mutations who exhibit CF-like features in the sweat gland and lung.

Toward wearable patch for sweat analysis

This work presents an inexpensive and disposable miniaturized electrochemical sensing platform, which was fabricated on a polyethylene terephthalate substrate for continuously monitoring the ion composition of sweat. The sweat patch consisted of an area for sweat collection, integrated reference electrode and an array of screen printed AgCl electrodes for real-time monitoring chloride concentration in sweat. The printing process and thickness of the AgCl electrodes were optimized to enhance the stability of the electrode in fluid. The AgCl electrodes were stable for more than a week in fluid and had a chloride sensitivity of 58mV per decade. The reference electrode was successfully fabricated on the same patch by adding a polyhydroxyethylmethacrylate hydrogel layer on top of one of the AgCl electrode in the array and followed by conditioning in 3M KCl solution. The patch, which was worn on the chest of a test subject, was able to continuously monitor the chloride concentration in sweat of the test subject during an one hour treadmill exercise. Reasonable agreement was shown between the real-time measurements and control measurements of sweat samples, which were collected from the test subject and analyzed off-line.

Safety, pharmacokinetics, and pharmacodynamics of ivacaftor in patients aged 2–5 years with cystic fibrosis and a CFTR gating mutation (KIWI): an open-label, single-arm study

Ivacaftor has been shown to be a safe, effective treatment for cystic fibrosis in patients aged 6 years or older with a CFTR gating mutation. We aimed to assess the safety, pharmacokinetics, and pharmacodynamics of ivacaftor in children aged 2-5 years. In the two-part KIWI study, we enrolled children aged 2-5 years weighing 8 kg or more with a confirmed diagnosis of cystic fibrosis and a CFTR gating mutation on at least one allele from 15 hospitals in the USA, UK, and Canada. Participants received oral ivacaftor 50 mg (if bodyweight <14 kg) or 75 mg (if bodyweight ≥14 kg) every 12 h for 4 days in part A (to establish the short-term safety of doses for subsequent assessment in part B), and then for 24 weeks in part B (to assess safety and longer-term pharmacodynamics). Children could participate in both or just one part of the study. Primary outcomes were pharmacokinetics and safety, analysed in all patients who received at least one dose of ivacaftor. Secondary outcomes were absolute change from baseline in sweat chloride concentrations and bodyweight, body-mass index (BMI), and height Z scores, and pharmacokinetic parameter estimation of ivacaftor. This study is registered with ClinicalTrials.gov, number NCT01705145. Between Jan 8, 2013, and March 1, 2013, nine patients were enrolled onto part A of the study, all of whom completed the 4 day treatment period, and eight of whom took part in part B. Between June 28, 2013, and Sept 26, 2013, 34 patients were enrolled in part B, 33 of whom completed the 24 week treatment period. All patients received at least one dose of ivacaftor. Results of ivacaftor pharmacokinetics suggested that exposure was similar to that reported in adults (median Cmin were 536 ng/mL for the 50 mg dose; 580 ng/mL for the 75 mg dose; median ivacaftor AUC values were 9840 ng × h/mL and 10 200 ng × h/mL, respectively). Common adverse events in part B included cough (in 19 [56%] of 34 patients) and vomiting (in ten [29%]). Five (15%) patients had liver function test (LFT) results that were more than eight times higher than the upper limit of normal, four of whom had study drug interrupted, and one of whom had study drug discontinued. Six (18%) of 34 patients had seven serious adverse events; a raised concentration of transaminases was the only serious adverse event regarded as related to ivacaftor and the only adverse event that resulted in study treatment discontinuation. At week 24, in patients for whom we had data, sweat chloride had changed from baseline by a mean of -46·9 mmol/L (SD 26·2, p<0·0001), weight Z score by 0·2 (0·3; p<0·0001), BMI Z score by 0·4 (0·4, p<0·0001), and height Z score by -0·01 (0·3; p=0·84). Ivacaftor at doses of 50 mg and 75 mg seems to be safe in children aged 2-5 years with cystic fibrosis with a gating mutation followed up for 24 weeks, although the frequency of elevated LFTs suggests that monitoring should be frequent in young children, particularly those with a history of elevated LFTs. Results of an ongoing extension study assessing durability of these effects and longer-term safety are warranted. Vertex Pharmaceuticals Incorporated.

Diagnosis of cystic fibrosis with chloride meter (Sherwood M926S chloride analyzer®) and sweat test analysis system (CFΔ collection system®) compared to the Gibson Cooke method.

Sweat test with Gibson Cooke (GC) method is the diagnostic gold standard for cystic fibrosis (CF). Recently, alternative methods have been introduced to simplify both the collection and analysis of sweat samples. Our aim was to compare sweat chloride values obtained by GC method with other sweat test methods in patients diagnosed with CF and whose CF diagnosis had been ruled out. We wanted to determine if the other sweat test methods could reliably identify patients with CF and differentiate them from healthy subjects. Chloride concentration was measured with GC method, chloride meter and sweat test analysis system; also conductivity was determined with sweat test analysis system. Forty eight patients with CF and 82 patients without CF underwent the sweat test, showing median sweat chloride values 98.9 mEq/L with GC method, 101 mmol/L with chloride meter, 87.8 mmol/L with sweat test analysis system. In non-CF group, median sweat chloride values were 16.8 mEq/L with GC method, 10.5 mmol/L with chloride meter, and 15.6 mmol/L with sweat test analysis system. Median conductivity value was 107.3 mmol/L in CF group and 32.1 mmol/L in non CF group. There was a strong positive correlation between GC method and the other sweat test methods with a statistical significance (r=0.85) in all subjects. Sweat chloride concentration and conductivity by other sweat test methods highly correlate with the GC method. We think that the other sweat test equipments can be used as reliably as the classic GC method to diagnose or exclude CF.

False-Positive and False-Negative Sweat Tests: Systematic Review of the Evidence.

The measurement of sweat chloride concentration (sweat test) remains the gold standard for the diagnosis of cystic fibrosis (CF). For infants with a positive newborn screening test for CF, the sweat test is the next step to confirm the diagnosis. However, there are several reports in the literature regarding conditions that are known to be associated with false positive or false negative sweat tests. This systematic review of the literature summarizes the evidence available regarding these conditions. Interpretation of sweat test results should be done in the context of patient's clinical presentation and associated comorbidities. Additional research is needed for conditions that may affect sweat chloride concentrations.

Book Review: Oxford Respiratory Medicine Library: Cystic Fibrosis

Book review: Cystic Fibrosis by A. Horsley et al. http://ow.ly/UpIzP

Non-allergic asthma as a CFTR-related disorder

BackgroundCFTR dysfunction can be involved in CBAVD, pancreatitis or bronchiectasis. MethodsSubjects with cystic fibrosis-like disease, equivocal sweat chloride concentrations and no or one disease-causing CFTR mutation were investigated by intestinal current and/or nasal potential difference measurements. ResultsA subgroup of female patients who had been diagnosed to suffer from non-allergic asthma showed intermediary chloride concentrations in sweat test, normal chloride secretory responses in the intestine and an abnormal nasal potential difference with Sermet scores in the cystic fibrosis range. ConclusionNon-allergic asthma is a clinical entity that may be associated with CFTR dysfunction of the respiratory epithelium.

Sweat chloride concentrations in children with Idiopathic Nephrotic Syndrome.

Idiopathic Nephrotic Syndrome (INS) has been believed to cause a false positive elevation of sweat chloride concentrations, as measured by the sweat test. Sweat tests were done for 11 children with acute onset INS at admission and again while they were in remission, with results being compared to normal historical controls. The initial sweat chloride concentration for 10 patients was normal (mean16.7 ± 11.02 mmol/L) and 1 patient had inadequate collection. This latter patient and two others were excluded during follow-up because of diagnoses other than INS. Sweat test results for the eight INS patients during follow up remained unchanged when they were in remission (16.94 ± 7.88 mmol/L; P = 0.98; Wilcoxon Matched-Pairs Signed Rank Test). In comparing sweat chloride concentrations from INS patients to those from 20 historical control subjects, we found no significant differences (Mann-Whitney Test; initial vs. control P = 0.643; follow up vs. control P = 0.806). INS does not cause a false positive sweat test. Further studies should be done to objectively assess the conditions that have been reported to affect sweat chloride concentrations.

On sweat analysis for quantitative estimation of dehydration during physical exercise.

Quantitative estimation of water loss during physical exercise is of importance because dehydration can impair both muscular strength and aerobic endurance. A physiological indicator for deficit of total body water (TBW) might be the concentration of electrolytes in sweat. It has been shown that concentrations differ after physical exercise depending on whether water loss was replaced by fluid intake or not. However, to the best of our knowledge, this fact has not been examined for its potential to quantitatively estimate TBW loss. Therefore, we conducted a study in which sweat samples were collected continuously during two hours of physical exercise without fluid intake. A statistical analysis of these sweat samples revealed significant correlations between chloride concentration in sweat and TBW loss (r = 0.41, p <; 0.01), and between sweat osmolality and TBW loss (r = 0.43, p <; 0.01). A quantitative estimation of TBW loss resulted in a mean absolute error of 0.49 l per estimation. Although the precision has to be improved for practical applications, the present results suggest that TBW loss estimation could be realizable using sweat samples.