Tuesday, May 1, 2007

How a Sweat Test Works

It all started with a heat wave in the first half of the last century. A New York area hospital experienced an unusually high number of cystic fibrosis patients requiring medical attention for severe dehydration. Doctors treating the patients hypothesized that something about the sweat of a CF patient was abnormal.

To test this hypothesis, the sweat test was developed. Medical professionals soon discovered that people with CF can have up to 5 times the amount of salt in their sweat as people without the condition. For this reason, the “sweat test” has a long standing history of being the primary diagnostic procedure when cystic fibrosis is suspected.

The sweat test can be performed on an infant as young as 2 days old. However, it should be noted that the results of a sweat test obtained at such an early age may not be conclusive. In such cases, the test needs to be repeated, or other diagnostic methods (such as genetic testing) may be recommended.

The procedure of a sweat test is simple, and proceeds in the following way:

1. A piece of gauze that has been soaked in a chemical called Pilocarpine is applied to an area of skin on the patient’s arm or thigh. This chemical induces sweat. Another piece of gauze, soaked in sodium chloride solution (salt water) is also applied.

2. Electrodes are then attached to the gauze pads, and a small amount of current is applied.

3. After 5 to 10 minutes, the electric current is stopped, and the pads are removed.

4. A fresh piece of gauze is used to soak up the sweat that was produced at the site exposed to Pilocarpine. The pad is left there for approximately 30 minutes.

5. Once enough time has passed, the pad is removed and placed into a collection tube for analysis.

6. Analysis of the gauze involves weighing it to determine the volume of sweat produced. The concentration of salt in the sweat is also analyzed.

Depending on the lab, results are made available to the patient (or parents) within a day or two. One of the distinguishing symptoms of cystic fibrosis is the abnormally high loss of salt through perspiration. For this reason, the results of the sweat test focus on the amount of salt in the accumulated sweat on the gauze pad. Concentration is measured in milimoles per liter and expressed as (mmol/L). The ranges of results of a sweat test are:

40 mmol/L and below = NORMAL (unlikely CF)

40 – 60 mmol/L = BORDERLINE (possible CF)

60 mmol/L or above = ABNORMAL (likely CF)

In the case of borderline or abnormal results, a repeat test may be performed at a later date. Alternatively, genetic testing may be done to arrive at a firmer conclusion. Abnormal values obtained from the sweat test strongly indicate cystic fibrosis; however, there are other conditions that must be ruled out in order for the diagnosis to be final.

To date, the most reliable way to diagnose CF is analysis of the sweat for amount of chloride (salt) it contains.


Additional resources:

Cystic Fibrosis Foundation--The Sweat Test

About Lung Diseases--Sweat Testing

Cystic Fibrosis Disease Profile

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