Sweat Test For Cystic Fibrosis

Overview of Cystic Fibrosis

Cystic fibrosis is a serious genetic disorder affecting 1 in every 4,000 people in the United States. It is caused by a mutation in the CFTR gene which encodes the protein cystic fibrosis transmembrane conductance regulator (CFTR). This protein is found in many tissues including the sweat glands, airways, lungs, gastrointestinal tract, and pancreas. CFTR is a component of the cell membrane, specifically in ion channels where ions like chloride, iodide, and thiocyanate move. Having two copies of mutant CFTR gene (one from mom and another from dad) will result to the production of defective CFTR protein or no CFTR at all. Defective or absent CFTR will produce abnormal ion channels that cannot regulate the proper movement of ions.

People with cystic fibrosis have more salty sweats than normal people. It is because defective chloride channel in the sweat gland prevents the reabsorption of chloride ions from sweat. The increased concentration of chloride ions in sweat is followed by outside movement of sodium ions.  This happens when negative chloride ions attract positive sodium ions to make salt (NaCl). Oppositely charged ions tend to move in the same direction. When excess chloride ions remain in the skin/sweat, excess sodium ions remain there as well.

Besides salty sweat, another symptom of cystic fibrosis is accumulation of thick, sticky mucus. This symptom is caused again by abnormal ion channels in airways. Thick, sticky mucus promotes growth of pathogenic bacteria which causes serious lung infection. In addition to lung infection, cystic fibrosis also causes frequent coughing, and shortness of breath.

Other symptoms of cystic fibrosis are poor growth and poor weight gain despite normal appetite. These symptoms can be due to the poor absorption of nutrients in the intestine and frequent infection. Infertility and coagulation disorders are other symptoms observed in people with the disease.
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What is Sweat Test?

Sweat test or sweat chloride test is a diagnostic procedure for cystic fibrosis.  This test measures the amount of sodium and chloride in sweat. As mentioned earlier, people with cystic fibrosis have higher concentration of sodium and chloride in their sweats than normal people.

Sweat Test Procedure

Sweat collection can be done in the skin on the forearm or at the legs. The skin is first washed and dried. Then a chemical called pilocarpine is applied to the skin. Next, an electrode is attached to the skin. A small electric current is delivered through this electrode to help pilocarpine stimulate sweating. The electric current is not painful but can give a tingling or warm sensation.

After applying current for 5-10 minutes, the skin is wiped dry and a piece of specialized filter paper covered with plastic (called sweat patch) is placed on in to collect sweat. Besides filter paper, a gauze pad or plastic coil can also be used to collect sweat. The collection of sweat can last from 30 minutes to 1 hour.  The filter paper is sealed in a plastic bottle and sent to the laboratory for examination. An experienced technician will measure the amount of chloride/sodium in the sample of sweat.

Sweat Test Result Interpretation

A sweat chloride concentration of greater than or equal to 60 milliequivalent per liter (mEq/L) is considered abnormal and may indicate cystic fibrosis. A genetic testing for the mutant CFTR genes will confirm cystic fibrosis. Two copies of the mutant genes are needed to develop cystic fibrosis. The patient will be tested for 23 common gene mutations out of almost 1000 possible mutations.

The normal amount of chloride in sweat is less than or equal to 39 mEq/L. A result between 40-59 mEq/L (considered the borderline) does not give a clear diagnosis, so further testing is needed.

Take note that normal value ranges may vary slightly among different labs. Talk to your physician about the meaning or interpretation of your sweat test result.

What Affects The Results?

There are diseases that can raise chloride levels in sweat giving false positive results. These are pancreatic infection, malnutrition, lipid storage disease, glycogen storage disease, adrenal insufficiency, hypothyroidism, hyperthyroidism, nephrogenic diabetes insipidus, Addison’s disease, and ectodermal dysplasia. These diseases, however, have symptoms different from cystic fibrosis.

Edema or buildup of excess serous fluid in tissues can result in a false positive result.[ad#afterpost]

References

• Sweat electrolytes – MedlinePlus Medical Encyclopedia
• Sweat Test – WebMD
• Sweat Chloride Test – Lab Tests Online
• Sweat Chloride Test for Cystic Fibrosis – MedicineNet