Sweat Test

The sweat test is a quantitative measurement of electrolyte concentrations (primarily sodium and chloride) in sweat, usually performed using pilocarpine iontophoresis (pilocarpine is a sweat inducer). Although this test is primarily used to confirm cystic fibrosis (CF) in children, it's also performed in adults to determine if they're homozygous or heterozygous for CF.

Purpose

• To confirm CF

Patient preparation

• Explain the test to the child (if he's old enough to understand), using clear, simple terms.
• Inform the child and his parents that there are no restrictions on diet, medication, or activity before the test.
• Tell the child who'll perform the test and where and that it takes 20 to 45 minutes (depending on the equipment used).
• Tell him he may feel a slight tickling sensation during the procedure but won't feel any pain.
• Encourage the parents to assist with preparations and to stay with their child during the test. Their presence will minimize the child's anxiety.

Equipment

Analyzer, two skin chloride electrodes (positive and negative), distilled water, two standardizing solutions (chloride concentrations), 2" X 2" sterile gauze pads (kept in airtight container), pilocarpine pads, forceps (for handling pads), straps (for securing electrodes), gram scale, normal saline solution.

Procedure and posttest care

• Wash the area that will undergo iontophoresis with distilled water and dry it. (The flexor surface of the right forearm is commonly used or, when the patient's arm is too small to secure electrodes [as with an infant], the right thigh.
• Place a gauze pad saturated with premeasured pilocarpine solution on the positive electrode; place the pad saturated with normal saline solution on the negative electrode.
• Apply both electrodes to the area to undergo iontophoresis, and secure them with straps. Lead wires to the analyzer are given a current of 4 mil­liamperes in 15 to 20 seconds. Iontophoresis will continue at 15- to 20­second intervals for 5 minutes.
• Try to distract the child with a book, television, a toy, or another diversion if he becomes nervous or frightened during the test.
• Remove both electrodes after iontophoresis.
• Discard the pads, clean the skin with distilled water, and then dry it.
• Using forceps, place a dry gauze pad or filter paper (previously weighed on a gram scale) on the area that underwent iontophoresis.
• Cover the pad or filter paper with a slightly larger piece of plastic, and seal the edges of the plastic with waterproof adhesive tape.
• Leave the gauze pad or filter paper in place for about 30 to 40 minutes. (The appearance of droplets on the plastic usually indicates induction of an adequate amount of sweat.)
• Remove the pad or filter paper with the forceps, place it immediately in the weighing bottle, and insert the stopper in the bottle. (The difference between the first and second weights indicates the weight of the sweat specimen collected. )
• Wash the area that underwent iontophoresis with soap and water, and dry it thoroughly. If the area looks red, reassure the patient that this is normal and will disappear within a few hours.
• Tell the patient or his parents that he may resume his usual activities.

Precautions

• Always perform iontophoresis on the right arm (or right thigh) rather than on the left.
• Never perform iontophoresis on the chest, especially in a child, because the current can induce cardiac arrest.
• Use battery-powered equipment to prevent electric shock if possible.
• Stop the test immediately if the patient complains of a burning sensation, which usually indicates that the positive electrode is exposed or positioned improperly. Adjust the electrode and continue the test.
• Make sure at least 100 mg of sweat is collected for analysis.
• Carefully seal the gauze pad or filter paper in the weighing bottle, and send the bottle to the laboratory at once.

Reference values

Normal sodium values in sweat range from 16 to 46 mmol/L or mEq/liter. Normal chloride values from 8 to 43 mmol/L.

Abnormal findings

Sodium levels of 75 mmol/L or more and chloride levels of 79 mmol/L or more are suggestive of CF.

Only a few conditions other than CF result in elevated sweat electrolyte levels - most notably, untreated adrenal insufficiency, as well as type I glycogen storage disease, vasopressin-resistant diabetes insipidus, meconium ileus, and renal failure. In women, sweat electrolyte levels fluctuate cyclically; chloride concentrations usually peak 5 to 10 days before onset of menses, and most women retain fluid before menses. Men also show fluctuations (up to 70 mmol/L). However, CF
is the only condition that raises sweat electrolyte levels above 80 mmol/L.

Interfering factors

• Dehydration or edema, especially in the area of collection
• Failure to obtain an adequate amount of sweat, a common problem in neonates
• Presence of pure salt depletion, common during hot weather (possible false-normal)
• Failure to clean the skin thoroughly or to use sterile gauze pads (possible false-high)
• Failure to seal the gauze pad or filter paper carefully (possible false-high electrolyte levels due to evaporation)