Corruption of sweat test results possible?

Question

Hello,

Is it possible that the sodium chloride value in a sweat test gets compromised 1. by cortisone and 2. by food (a lot of salt sticks)? After our first test, the value was at 62 mmol/l sodium chloride content. According to the hospital, the threshold is at 60-80 mmol/l.

Our son has been inhaling a spray containing cortisone for four weeks. In addition, two weeks ago an ointment containing cortisone was applied.

Answer

Hello,

You are asking whether cortisone administered as an inhalant over four weeks and concurrent treatment of the skin with an ointment containing cortisone can alter your son’s sweat test results. At the same time, you are asking whether the intake of larger amounts of salt while eating salt sticks can corrupt the result. Your son’s sodium chloride level was determined at 62 mmol/l.

Concerning the possible effect of cortisone on the sweat test results, it has to be said that highly dosed cortisone may alter the results in that there may be a slight decrease in the values. However, it is rather safe to say that this is not to be expected when taking a cortisone-containing inhalant, even if this is done in combination with the proper application of a cortisone ointment. It can happen, though, with administration of cortisone tablets or infusions over several days.

Severe dehydration (exsiccosis), which considerably affects a person’s general condition, can lead to false-positive (too high) sweat test results. This will usually not happen if a person eats salts sticks regularly and is feeling good with it. Your son’s kidneys will be able to stabilize his salt balance even with the increased intake by increased salt excretion.

Concerning the performance of the sweat test, it is important that the test is done in an experienced center using the so-called pilocarpine ionotophoresis to measure the chlorid concentration in the sweat. Only if this standardized procedure is used, the values are suitable to exclude or make the diagnosis of CF; then the following standardized values pertain: under 40mmol/l chloride concentration in the sweat: negative, 40-59mmol/l: borderline, equal or over 60mmol/l: pahtologic.
So from your question we cannot be sure that the test had been performed in this way, as you mention only the sodium chloride concentration and a different thershold.
Therefore, you should first do another test according to those standards with repeat determination on both arms. If the result is borderline to pathologic, further tests should be done, which you should discuss with doctors at your CF centre.
Kind regards
Dr. H.-G. Posselt

16.08.2012