Sweat Chloride

What is a sweat test?

It is a test where we collect the sweat of the baby by the procedure as described below. This sweat after collection is analyzed for the concentration of chloride.

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Most patients with Cystic Fibrosis have high concentrations of chloride in their sweat.

In which conditions a sweat test is done?

  • If a baby is showing signs or symptoms of cystic fibrosis then sweat of the baby needs to be tested for chloride levels.
  • Symptoms suggestive of cystic fibrosis are recurrent wet/productive cough, recurrent lung infections, not gaining weight/failure to thrive, recurrent diarrhea, bronchiectasis, etc. For details about cystic fibrosis.

What preparation is required for a sweat test?

  • No special preparation is required for a sweat test. The child can eat, drink and continue to take any current medicines.
  • It usually takes an hour for a sweat test to be completed.

How is a sweat test done?

Sweat tests are a bit different from most of the laboratory tests which are done. Let’s us understand how this procedure is done: There are two parts of the test: the first involves collection of sweat and the second involves the analysis of the sweat to get the chloride levels.

For collection of sweat Quantitative pilocarpine iontophoresis is performed using the Gibson and Cooke’s method.

An area of skin on the arm is cleaned and dried. Next, two electrodes are attached with straps as shown in the photograph below. One of these contains a filter paper with pilocarpine, a medicine that makes the sweat glands produce sweat. A weak electric current pushes the medicine through the skin. After this is done, the electrodes are removed and the skin is cleansed again.

A filter paper is then attached to the clean skin surface in the area where the sweat glands were stimulated as shown in the photograph below. It’s taped to the skin to keep it from moving. The sweat is collected for 30 minutes.

In the second step sweat is extracted from the filter paper and sweat chloride levels are estimated in a analyzer.

At what age a sweat test can be done?

Sweat test can be done at any use.

When are the sweat test results available?

The sweat test results are usually available on the same day or the next working day.

Is the sweat test painful?

No. There are no needles or injections involved in the test. It is very easy to perform the test as described above.

Understanding sweat test results?

The table below explains the sweat results. In case the sweat chloride levels are high then the test needs to be repeated. Two sweat results are taken as positive.

Age Sweat Chloride LevelsInterpretationLikelihood of Cystic Fibrosis
For infants up to 6 months of ageEqual to or less than 29 mmol/LNegativeCF is very unlikely
Between 30 – 59 mmol/LIntermediateCF is possible
Greater than or equal to 60 mmol/LPositiveCF is likely to be diagnosed
For babies older than 6 months of ageEqual to or less than 29 mmol/LNegativeCF is very unlikely
Between 30 – 59 mmol/LIntermediateCF is possible
Greater than or equal to 60 mmol/LPositiveCF is likely to be diagnosed