Non conclusive sweat test

Question

Hello, following a hospitalization for bronchitis with severe oxygen-dependent asthma crisis and analyzes of positive staphylo and pseudomonas in sputum, we carried out a sweat test. Unfortunately the data are not usable because not enough sweat. We have to do it again. My question is: if my daughter does not produce sweat again, what solutions do we have to establish or not for the diagnosis of CF?

Answer

Hello,

First, check that the optimum sweat test conditions are met. The sweat test must be carried out by persons who are familiar with the technique, with the appropriate equipment and by following a rigorous procedure both for the stimulation of sweating and for the determination of the concentration of salt (sodium chloride) in the sweat. The correct hydration of the patient and the good condition of the skin where the test is performed must be checked. Sweating should be stimulated by a small amount of fluid (pilocarpine) applied to a small area of the skin of a limb and applying a small (not painful or dangerous) electrical current to improve product penetration. The sweat is collected by capillarity in a small coil of plastic pipe which makes it possible to check that the quantity is sufficient. The result of the salt dosage should be announced the same day as a result by the physician of the CF Center who has got the expertise in the disease and experience of the announce of the diagnosis.

Given the difficulty in obtaining sufficient sweat in neonates, it is recommended that the test be carried out in infants aged over 3 weeks and weighing more than 3 kg using a suitable device for this age.

Diagnosis can also be made by identifying two mutations of the CFTR gene known to be causing CF on a sample of a few ml of blood.

In very rare cases where neither the sweat test (insufficient sweat quantity or limit concentration of sweat salt) nor the genetic test make it possible to conclude (identification of a single mutation or of two mutations of which only one is known as causing CF), other examinations can be carried out in some specialized centers: measurement of nasal potential difference or measurement of transepithelial currents on rectal mucosa (by non-painful biopsy-aspiration).

Hope this answer can help.
Best wishes
Gilles RAULT, MD, Roscoff CF Center

14.02.2017