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Does sweat volume influence the sweat test result?
  1. Shmuel Goldberg1,
  2. Shepard Schwartz2,
  3. Mimouni Francis2,
  4. Halina Stankiewicz3,
  5. Gabriel Izbicki4,
  6. Elie Picard1
  1. 1Department of Pediatric Pulmonology, Shaare Zedek Medical Center, Hebrew University Medical School, Jerusalem, Israel
  2. 2Department of Pediatrics, Shaare Zedek Medical Center, Hebrew University Medical School, Jerusalem, Israel
  3. 3Gastroenterology Institute, Shaare Zedek Medical Center, Hebrew University Medical School, Jerusalem, Israel
  4. 4Pulmonary Institute, Shaare Zedek Medical Center, Hebrew University Medical School, Jerusalem, Israel
  1. Correspondence to Shmuel Goldberg, Pediatric Pulmonology, Shaare Zedek Medical Center, PO Box 3235, Jerusalem 91301, Israel; sgoldberg{at}szmc.org.il

Abstract

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 µl).

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 µl 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 µl may represent an artefact related to the low volume.

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Footnotes

  • Competing interests None.

  • Ethics approval This study was conducted with the approval of the Ethics Committee of Shaare Zedek Medical Center, Hebrew University Medical School, Jerusalem, Israel.

  • Provenance and peer review Not commissioned; externally peer reviewed.

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