STANDARD OPERATING PROCEDURE
USING THE Sherwood Scientific Model 926S CHLORIDE ANALYSER
CLINICAL INDICATION
The sweat test is the gold standard for the diagnosis of cystic fibrosis, which would be confirmed by elevations in the sweat electrolytes.
PATIENT SAMPLE
Sample type may be serum, plasma, urine sweat or CSF.
The known weight of sweat from patients having undergone iontophoretic stimulation with pilocarpine will require elution in 10mls of distilled water before analysis.
NB If sodium is analysed on the same sample then the diluent will be 100 ppm Lithium carbonate in distilled Water
CALIBRATOR
Sherwood Multical containing l40mmol/l sodium, l00mmol/l chloride. Catalogue Number:47852000
EQUIPMENT
Sherwood Scientific Model 926S Analyser ( Part 92611005)
Positive displacement pipette.
Adjustable 1 -5 ml pipette.
REAGENTS
Combined acid buffer. Catalogue Number 00156608 Sherwood Scientific (01223 243444)
Polish for silver electrodes. (Catalogue Number 00156205) Sherwood Scientific
Diluent prepared from (Cat 00156222) Sherwood Scientific
PROCEDURE
1.Switch on the instrument on - rear panel of instrument, and allow 15 minutes warm up to meet the stated specification. Check that all the indicators are illuminated and that the digits on the readout are illuminated.
2.Select the '20µ1' sample volume on the instrument.
3.Condition and calibrate with Calibrator
4. Keeping the electrodes immersed, pipette 1 ml of sweat eluate into the beaker and depress the titrate button. When the stirrer stops, note the reading on the display. Repeat this cycle for each test
5.When the 'change reagents' indicator comes on, lower the beaker platform and empty the beaker contents. Rinse with distilled water and dry with tissues.
6.Continue with fresh combined acid buffer. Recondition the electrodes and proceed with checking the calibration.
7.When determinations are complete, remove the beaker and allow the electrodes to dry naturally.
RESULTS AND CALCULATION
Sweat chloride in mmol/l = Reading of Analyser x Weight of sweat in grammes
0.200
Round results off to the nearest whole number and when the controls have been verified as within the acceptable range (+1- 2SD) report results to the requesting clinicians and in the laboratory computer.
Abnormal results, equivocal results and patients that yield small volumes of sweat are always repeated.
REFERENCE RANGE
Sweat chloride:
| Less than 40 mmol/l | 40-60mmol/l | greater than 60 mmol/l |
| NEGATIVE | EQUIVOCAL | Consistent with CF. POSITIVE |
Sweat chloride concentrations should be interpreted with regard to the clinical presentation, patient's age and the knowledge that rare CFTR mutations are associated with borderline or negative sweat chloride results.
A sweat chloride concentration of more to 60 mmol/L is consistent with the diagnosis of CF but the result must be interpreted in the context of the patient's age and clinical picture by a physician knowledgeable about CF. Some data suggests that in infants younger than 3 months of age, a sweat chloride concentration of more than 40 mmol/L is highly suggestive of a diagnosis of CF. The diagnosis of CF should be made only if there is an elevated sweat chloride concentration (>60 mmol/L) on two separate occasions in a patient with one or more clinical features consistent with the CF phenotype or a history of CF in a sibling. Because sweat sodium concentrations of 60 to 80 mmol/L can be seen in individuals with diseases other than CF measurement of sodium alone is not recommended.
However, in some cases, especially those with borderline sweat results, measurement of both sodium and chloride concentrations can be helpful In patients with CF, both analytes should be proportionately elevated (within 15 mmol/I-). and the chloride/sodium ratio is almost always greater than 1.0 A sweat chloride concentration of me than 160 mmol/L is physiologically impossible and suggests an error in collection or analysis. Tests with such results should be repeated.
Sherwood Scientific manufactures the Model 420 Flame Photometer and has a protocol that allows for the measurement of chloride and sodium from a single Sweat Sample. This requires high levels of accuracy and the Model 420 with its Lithium Internal standard is ideal for this task.
A combined protocol now means that a single eluate, taken in a lithium blank, can be used both for chloride and sodium (and potassium if required).
