Measuring Sodium Concentration in Sweat Collected on Gauze or Filter Paper

 

Reagents and supplies:

(1) diluent as supplied by manufacturer

(2) sodium calibrators: 10, 20 and 40 mmol/L, with greater concentrations required if patient sodium is > 40 mmol/L

(3) low and high sodium controls: 30 and 100 mmol/L

(4) preweighed gauze or filter paper

(5) paraffin wax film

(6) clean, dry vials

(7) powder free gloves

 

Instrumentation:

(1) sweat collection instrumentation for providing pilocarpine iontophoresis

(2) sodium analyzer (flame photometer, other)

(3) accurate analytical balance sensitive to 0.0001 g

 

Specimen collection:

(1) Using preweighed gauze or filter paper is placed over the site of the positive electrode and covered with paraffin film to prevent evaporation.

(2) For complete details on specimen collection, refer to the NCCLS document.

(3) After sweat collection, the gauze or filter paper is removed and reweighed.

(4) The increase in weight corresponds to the volume of the sweat collected. The relative density of sweat ranges from 1.001 to 1.008.

(5) The sweat is then eluted from the gauze or filter paper using a volume of diluent.

 

Test vial types, prepared in duplicate:

(1) gauze or filter paper blank vial

(2) calibrator vials (one for each calibrator)

(3) test vial

(4) low control vial

(5) high control vial

 

Each vial contains:

(1) 10 mL of diluent

(2) 50 µL of fluid based on vial type:

(2a) test eluate for patient vial

(2b) control eluate for control vials

(2c) gauze eluate in gauze or paper blank vial, or

(2d) calibrator solution for calibrator vial (one for each calibrator)

 

Sodium measurement:

(1) The calibrators are read to generate a calibration curve for a 1:200 dilution (50 µL in 10 mL)

(2) Sodium is measured on each test vial type prepared in duplicate.

(3) The result for each vial type is the average of the two readings.

 

Quality Control:

(1) amount of sweat collected: At least 0.075 grams of sweat should be collected when 2x2 inch gauze is used.

(2) accuracy of micropipette: The micropipette should be verified as accurate.

(3) controls: The low and high sodium controls should give results comparable to formulated concentrations.

(4) technique: The operator must ensure that no artifact is introduced during the procedure. The gauze or filter paper must not be handled without gloves and the gloves should be powder free. Paraffin wax film is used to minimize evaporation during the specimen collection period.

 

sweat sodium in mmol/L for test sample =

= A * B * 1/200

 

where:

• A = ((sodium in test vial in mmol/L) - (sodium in gauze or paper blank vial in mmol/L))

• B = ((mL of dilutent used to elute gauze or filter paper) + (grams of sweat collected added)) / (grams of sweat collected or control) if test sample

• 1/200 is the dilution of the sample (50 µL added to 10 mL)

 

sodium in mmol/L in control

= ((sodium in control vial in mmol/L) - (sodium in gauze or paper blank vial in mmol/L))

 

Interpretation:

• A sodium concentration > 60 mmol/L is consistent with the diagnosis of cystic fibrosis.

• A sodium concentrations from 40-60 mmol/L is considered borderline.

• A sodium concentrations < 40 mmol/L is negative.

• Some unaffected adults may have sodium concentrations > 60 mmol/L, and several disorders other than cystic fibrosis are associated with an increased sweat electrolyte concentration.

• The diagnosis of cystic fibrosis requires more than one sodium or chloride determination and should be based on the patient's clinical picture by a physician knowledgeable about cystic fibrosis.