Does sweat volume influence the sweat test result?

OBJECTIVE: Low volume sweat samples are considered unreliable for the diagnosis of cystic fibrosis, based on the assertion that sweat conductivity and chloride are reduced at lower sweating rates. We aimed to re-evaluate the relationship between sweat volume and test results. DESIGN: We reviewed all sweat tests performed in our institution to assess the relationship between sweat volume and conductivity, and between sweat volume and sweat chloride. We also compared results between pairs of sweat tests taken simultaneously from a single patient, one with sweat volume below and the other above the currently accepted minimum volume (15 microl). RESULTS: A weak inverse relationship between sweat volume and sweat conductivity was found (n=1500, R2=0.105, p<0.001). There was no correlation between sweat volume and sweat chloride (n=463, R2=0.002, p>0.05). In discordant pairs (one below and one exceeding the accepted minimum volume), the mean test result in the low volume sample was slightly higher than its counterpart. In 76 such pairs, mean conductivity was 41.1+/-14.6 mmol/l in the lower volume sample, compared with 36.8+/-16.0 mmol/l in the higher volume sample (p<0.001). Similarly, in 33 of the pairs, mean sweat chloride was 28.4+/-15.7 mmol/l in the lower volume sample compared with 25.1+/-15.2 mmol/l in the higher volume sample (p=0.004). CONCLUSION: A normal sweat conductivity and/or chloride value from a sweat volume <15 microl in a patient whose clinical symptoms are not very suggestive of cystic fibrosis, renders this diagnosis unlikely. In contrast, elevated sweat chloride or conductivity measured from a sample whose volume is <15 microl may represent an artefact related to the low volume.

Exocrine pancreatic function testing in patients with cystic fibrosis and pancreatic sufficiency: a correlation study.

OBJECTIVES: Most patients with cystic fibrosis (CF) have pancreatic insufficiency; however, 15% of the patients are pancreatic sufficient (PS). Several laboratory tests have been developed to distinguish between pancreatic insufficiency and PS. The gold standard to determine pancreatic function apart from direct pancreatic stimulation test is the 72-hour fecal fat excretion, expressed as coefficient of fat absorption (CFA). The aim was to test the correlation between 2 other tests, fecal elastase-1 and serum immunoreactive trypsinogen (IRT), as compared with fecal fat excretion. PATIENTS AND METHODS: 21 patients with CF-PS performed the 3 tests of fecal fat excretion, fecal elastase-1, and IRT. Correlation between the tests was evaluated by the kappa statistics test, sensitivity and specificity, and positive and negative predictive values. RESULTS: CFA was abnormal in 5 patients, elastase was <200microg/g in 4 patients, and IRT was <20 ng/mL in 2 patients. The correlation between CFA and IRT was negative(kappa=-0.154), and between CFA and fecal elastase-1 was poor (kappa=0.213). The sensitivity, specificity, and positive and negative predictive values of IRT versus CFA were 0%, 88%, 0%, and 78%, and for fecal elastase-1 were 40%, 81%, 40%, and 81%, respectively. CONCLUSIONS: In CF-PS, poor correlation was found between IRT, fecal elastase-1, and CFA, therefore neither fecal elastase-1 in the stool nor IRT in the serum reaches the sensitivity or the specificity of the fecal fat excretion. Thus, fecal fat excretion is required in patients with CF for evaluation of pancreatic function.

Vitamins A and E and pulmonary exacerbations in patients with cystic fibrosis.

BACKGROUND: Increased levels of oxidative stress result in pulmonary damage contributing to the development of chronic lung disease in cystic fibrosis (CF). The aim of this study was to investigate the longitudinal effect of serum vitamin A and E levels on the incidence of pulmonary exacerbations in pancreatic insufficient (PI) and pancreatic sufficient (PS) patients with CF. MATERIALS AND METHODS: Patient records were retrospectively examined over a 3-year period and serum vitamin A and E levels were retrieved. Subsequently, levels of vitamin A and E were prospectively measured over a 2-year period at the onset of intravenous antibiotic therapy for acute exacerbation and at the first recovery visit. RESULTS: Retrospectively, 597 pulmonary exacerbations were identified in 102 patients, 74 PI and 28 PS, with a mean age of 11.1 +/- 6.4 years (range, 1.5-27 y). An increased number of exacerbations was directly correlated with lower vitamin A and E levels, even within the normal range. Prospectively, 62 exacerbations were analyzed (43 PI patients and 19 PS patients). At onset of exacerbation, vitamin A and E levels were reduced in the PI patients (P < 0.001; P < 0.001) and the PS patients (P < 0.005; P < 0.07). CONCLUSIONS: Reduced serum levels of vitamin A and E even in the normal range are associated with an increased rate of pulmonary exacerbations in CF. Further studies are required to confirm the necessity of supplementation of vitamins A and E to PS patients.