- It is good clinical practice to prepare the patient and, where appropriate, parent effectively before testing.
Informed consent should be obtained in accordance with local policy.
Pretest information appropriate for the individual should include why the test is being done, how it will be performed, risks associated with the test, what the subject will experience, and contact details regarding the testing and final result. An example leaflet for patients/parents is provided (see Appendix document 1 of the original guideline document).
|
C |
|
| Subject Suitability |
|
|
- Sweat tests can be performed after 2 weeks of age in infants greater than 3 kg who are normally hydrated and without significant systemic illness
|
C |
|
- Sweat testing can be attempted in term infants after 7 days of age if clinically important, but will need repeating if insufficient quantity of sweat is collected.
|
C |
|
- Sweat tests should be delayed in subjects who are dehydrated, systemically unwell or who have eczema affecting the potential stimulation sites.
|
C |
|
- Sweat tests should be delayed in subjects who are oedematous and/or on systemic corticosteroids.
|
C |
|
- Sweat tests should not be performed in subjects who are on oxygen by an open delivery system. This would not apply to an infant in headbox or on nasal prong oxygen.
|
C |
|
- Sweat tests can be performed in subjects on flucloxacillin.
|
C |
(Williams et al., 1988) |
| Sweat Collection |
|
|
- The flexor surface of either forearm is the preferred site for sweat collection. Consideration may be given to other sites if both arms are eczematous, too small or otherwise unsuitable. Other sites used successfully include the upper arm, thigh and back.
|
C |
|
- Great care must be taken at all stages of the procedure to avoid contamination (see example standard operating procedure [SOP] in the original guideline document).
|
C |
|
- In response to a sweat test request it is sufficient to carry out one sweat collection only.
|
Not Graded*
|
(Reynolds, personal communication) |
- The power supply used must be battery powered and should include a safety cutout.
- Monitoring of the current must be carried out throughout iontophoresis where possible. Wescor systems from model 3600 onwards have no ammeter but have an appropriate safety cut out system.
- The power supply and electrodes must be regularly checked, maintained and records kept.
- Electrical safety of all power supplies must be checked annually.
|
C |
|
- Electrodes should be of a suitable size and curvature to fit snugly on the patient's limb.
- They are most commonly made of copper or stainless steel.
- Electrodes should be firmly secured in position to the electrolyte support pads or gels using straps that are adjustable to fit the patient (e.g. Velcro or rubber).
- Electrodes must be regularly cleaned and inspected, and discarded if they show pitting or irregularities.
|
C |
|
- Selection of new equipment, and maintenance of existing equipment, must comply with Clinical Pathology Accreditation (CPA) (or equivalent standard).
|
C |
|
|
Electrolyte Solutions
- Aqueous solutions or Wescor gel discs containing pilocarpine nitrate at 2-5 g/l are recommended for use at both electrodes. Alternative solutions (e.g. magnesium sulphate) may be used at the cathode.
|
B |
(Price & Spencer, 1977; Szabo, Kenny, & Lee, 1973) |
- Solutions containing sodium and/or chloride should be avoided because of the risk of contamination of the collection.
|
C |
|
- Unbuffered acid solutions should not be used because of the increased risk of burns.
|
B |
(Schwarz, Sutcliffe, & Style, 1968) |
- Electrolytes used for iontophoresis must either be obtained as part of a medical device (e.g. Wescor Pilogel Discs) or from a recognised manufacturer of unlicensed medical products. Solutions must not be produced in-house by hospital laboratories.
|
C |
|
|
Electrolyte Supports
- Suitably thick pads must be used for the electrolyte solutions to minimise the risk of acid burns.
|
B |
(Schwarz, Sutcliffe, & Style, 1968) |
- Pads of Hospital Lint BPC Plain 500 gram folded to provide 4-8 thicknesses (greater than 1 cm thick) are recommended as an electrolyte reservoir with filter paper collection systems. The pad should be at least 1 cm larger than the electrode in all directions to prevent electrode-skin contact. It may be incorporated into sewn pockets designed to contain the electrode and prevent skin contact. The pads should be saturated by soaking in the electrolyte solution before application to the patient's skin.
|
Not Graded* |
|
- Hybrid systems (e.g. Wescor electrodes with aqueous electrolyte solutions, or Wescor gel discs used with non-Wescor electrodes) should not be used.
|
Not Graded* |
|
|
Iontophoresis - time, current
- When aqueous electrolyte solutions are applied on pad supports to a current of 0.5 mA should be applied, and increased gradually to a maximum of 4 mA. Once 4 mA is attained the current should be maintained for a minimum of 3 minutes and a maximum of 5 minutes. Longer times should be necessary to increase sweat production, provided good electrical contact is maintained, by use of well maintained electrodes and suitably saturated pads.
|
B |
(Instruction manual, 1979; Price & Spencer, 1977; Gibson & Cooke, 1959; Webster & Barlow, 1981; Webster, 1983; Kirk et al., 1983) |
- When Wescor systems are used, the manufacturer's current and time recommendations should be followed. This will depend on the specific model used.
|
C |
|
- For both systems, the patient must be kept under close supervision throughout the iontophoresis period.
|
C |
|
|
Medium of Collection
- During collection, sweat must be protected from contamination and evaporation (see example SOP in the original guideline document).
|
C |
|
- Sweat should be collected onto preweighed sodium chloride free filter paper or Wescor disposable collectors.
|
C |
|
- The size of the filter paper should be approximately equal to the area stimulated (i.e. the size of the electrolyte support pads).
|
C |
|
- Filter paper should be covered with a sheet of impervious material at least 1 cm larger in all dimensions than the filter paper.
|
C |
|
- The impervious material must be completely sealed to the skin surface using a suitable adhesive tape.
|
C |
|
- Filter paper and the inner side of the impervious material must never come into direct contact with the operator's hands.
|
C |
|
- Wescor collectors should be used according to the manufacturer's instructions, taking precautions to avoid direct contact of the sweat collecting surface with the operator's hands.
|
C |
|
|
Collection Time
- Sweat should be collected for not more than 30 minutes and not less than 20 minutes.
|
B |
(Reynolds, personal communication ; Price & Spencer, 1977; Shwachman & Mahmoodian, 1966; Webster & Quirante, 2000; Webster & Barlow, 1981; Kirk et al., 1983; Simmonds et al., 1989; Gibson & di Sant'Agnese, 1963; Schwarz, Simpson, & Ahuja, 1977; Hjelm, Brown, & Briddon, 1986)
|
- The Orion electrode should not be used.
|
B |
(Price & Spencer, 1977; Denning et al., 1980)
|
Sweat Analysis
Pre-analytical
|
|
|
|
Storage before analysis
- Throughout sweat collection (including transport and analysis) every effort should be made to minimise evaporation of the sample.
|
C |
|
- If storage is necessary before analysis, sweat collections on paper pads should be kept at 4 degrees Celsius for a maximum of 3 days and in appropriately sized, air tight containers which do not allow leakage or evaporation.
|
B
|
(Legrys, 1993)
|
- Liquid sweat from Macroduct collections can be stored in sealed macroduct tubing for up to 72 hours at 4 degrees Celsius. Haematocrit tubes sealed with plasticine are also suitable, providing an air gap is left between plasticine and sweat.
|
B |
(Kirk, personal communication, 2001; Fahie-Wilson & Freedman, 1995)
|
- Sweat may be collected at remote sites and transported to the laboratory for analysis provided there is attention to storage details.
|
B |
|
| Weighing |
|
|
- The same balance must be used throughout.
|
C |
|
- A balance sensitive to 0.0001 g must be used to weigh sweat.
|
C |
|
- Sweat collections onto paper pads should be weighed and analysed as soon as practicable.
|
C |
|
|
Definition of adequate sample
- The sweat secretion rate measured as an average rate over the collection period should not be less than 1 g/m2/min. Collections below this rate should not be analysed. Insufficient sweat collections should not be pooled. The full sweat test should be repeated.
|
B |
(Price & Spencer, 1977; Webster & Quirante, 2000; Kirk et al., 1983; Simmonds et al., 1989; Hjelm, Brown, & Briddon, 1986; Gibson & de Sant'Agnese, 1963)
|
| Analysis |
|
|
|
Elution of sweat from filter paper
- When sweat is collected onto filter paper (refer to section 3.2.1 of the original guideline document) it should be eluted for a minimum of 40 minutes.
|
C |
|
|
Analytes
- Sweat chloride concentration should be measured.
|
B |
(Green, Dodds, & Pennock, 1985; Hall, Stableforth, & Green, 1990; Gleeson & Henry, 1991, Kirk et al., 1992) |
- Sweat sodium must not be the only or primary analyte measured.
|
B |
(Green, Dodds, & Pennock, 1985; Hall, Stableforth, & Green, 1990; Gleeson & Henry, 1991, Kirk et al., 1992) |
- Sweat potassium measurement is not recommended.
|
B |
(Shwachman, Mahmoodian, & Neff, 1981)
|
- Sweat conductivity measurement for the investigation of cystic fibrosis (CF) requires further study. If conductivity is measured, sweat chloride should also be measured until the relative merits of conductivity have been established.
|
B |
(Heeley, Woolf, & Heeley, 2000; Mastella et al., 2000; Legrys, 2001; Heeley, Woolf, & Heeley, 2001; Webster, 2001) |
- Sweat osmolality measurement is not recommended.
|
B |
(Kirk, personal communication, 2001; Heeley, Woolf, & Heeley, 2000)
|
|
Methodology
- Colorimetry, coulometry and ion selective electrodes (ISEs) are satisfactory methods for analysis of sweat chloride.
|
B |
(Fahie-Wilson and Freedman, 1995; Finlay MacKenzie, 2001; Heeley, Woolf, & Heeley, 2000)
|
- Flame photometry or ion selective electrodes are satisfactory methods for analysis of sweat sodium.
|
B |
(Finlay MacKenzie, 2001; Barbour, 1991; Northall & York, 1995)
|
- Conductivity measurement using the Wescor equipment is a satisfactory method of analysis.
|
B |
(Finlay MacKenzie, 2001; Heeley, Woolf, & Heeley, 2000) |
|
Report format
The report format should include:
- Full patient identification
- Date and time of test and date and time of report
- Sweat weight/volume collected and minimum weight/volume acceptable for local sweat test parameters
- Analytical results (mmol/L)
It should be explicit on the report form which analyte(s) have been measured (i.e., chloride, sodium, conductivity [sodium chloride equivalent]).
- Reference ranges (see section 6 of the original guideline document)
- Interpretation of the results (see section 6 of the original guideline document)
- Recommendations for repeat testing if appropriate (see section 6.9 of the original guideline document)
|
C |
|
| Quality |
|
|
- Sweat which has been subject to evaporation and/or contamination must not be measured.
|
C |
|
- The analytical range of the methods used must cover the concentration ranges found in normals and subjects with CF.
|
C |
|
|
|
C |
|
- There must be an internal quality procedure (which differs from the calibration/standardisation procedure) at two concentrations (normal and intermediate or abnormal) for each analysis.
|
C |
|
- The analytical methods should each have a between batch coefficient of variation (CV) of 5% (or less) at a concentration of 40-50 mmol/L.
|
B |
(Heeley, Woolf, & Heeley, 2000; Hall, Stableforth, & Green, 1990; Hammond, Turcios, & Gibson, 1994; Kirk et al, 1992; Ayers, 2000; Taylor & James, 1996)
|
- The laboratory must participate in a suitable external quality assessment scheme.
|
C |
|
- If chloride and sodium concentrations are widely discrepant, the test should be repeated.
|
B |
|
- Results which are not physiological should be questioned (i.e. chloride or sodium > 150 mmol/L).
|
B |
(Schulz, 1969) |
- For conductivity a provisional upper physiological limit of 170 mmol/L may be used pending further evidence.
|
C |
|
- Failed sweat collections (i.e. insufficient weight or volume) should not exceed 10% of the tested population (excluding repeats and tests carried out in sick/very young patients). There should be a target of 5%.
|
C |
|
- Performance of sweat testing should be reviewed on a regular basis. This should include:
- Insufficient collections
- as % of total tests
- per operator
- Analytical failure rate (i.e. % outside accepted quality control (QC) range)
- External quality assessment performance
|
C |
|
The laboratory should work with clinicians to audit sweat test results, in particular repeat collections, diagnoses and outcome of positive and intermediate results on a regular basis (see section 8 of the original guideline document). |
C |
|
| Interpretation of sweat electrolytes |
|
|
|
The following definitions are recommended for interpretation:
- A sweat chloride concentration of >60 mmol/L supports the diagnosis of CF
|
B |
(Green, Dodds, & Pennock, 1985; Hall, Stableforth, & Green, 1990; Rosenstein, 1999; Rosenstein & Cutting, 1998; Farrell & Koscik, 1996; "Correlation between genotype and phenotype," 1993; Davis et al., 1983; di Sant'Agnese et al., 1953)
|
- Intermediate chloride concentration of 40-60 mmol/L is suggestive but not diagnostic of CF.
|
B |
(Green, Dodds, & Pennock, 1985; Hall, Stableforth, & Green, 1990; Rosenstein, 1999; Rosenstein & Cutting, 1998; Farrell & Koscik, 1996; Correlation between genotype and phenotype," 1993; Davis et al., 1983; di Sant'Agnese et al., 1953)
|
- A sweat chloride of less than 40 mmol/L is normal and there is a low probability of CF.
|
B |
(Green, Dodds, & Pennock, 1985; Hall, Stableforth, & Green, 1990; Rosenstein, 1999; Rosenstein & Cutting, 1998; Farrell & Koscik, 1996; "Correlation between genotype and phenotype," 1993; Davis et al., 1983; di Sant'Agnese et al., 1953)
|
- Sodium should not be interpreted without a chloride result.
|
B |
(Green, Dodds, & Pennock, 1985) |
- Pending further data on conductivity measurements a value below 60 mmol/L (NaCl equivalents) is unlikely to be associated with CF. Values above 90 mmol/L support a diagnosis of CF.
|
B |
(Hammonds, Turcios, & Gibson, 1994; Heeley, Woolf, & Heeley, 2000; Mastella et al., 2000).
|
- CF should not be diagnosed based on conductivity measurements alone.
|
B |
(Hammonds, Turcios, & Gibson, 1994; Heeley, Woolf, & Heeley, 2000; Mastella et al., 2000). |
|
Repeat Testing
- A repeat sweat test is recommended when the sweat test result is not in keeping with the clinical phenotype and/or genotype.
|
C |
|
|
Further Investigations
- Mutation analysis can be a useful diagnostic test, particularly in patients with a mild or atypical phenotype where sweat chloride concentration may be intermediate.
|
B |
(Augarten et al., 1995; Stern et al., 1978; Highsmith et al., 1994; Augarten et al., 1993; Strong et al., 1991; Gilfillan et al., 1998) |
- Nasal potential difference may be helpful as a confirmatory investigation for the diagnosis.
|
B |
(Rosenstein, 1999; Alton et al., 1987; Delmarco et al., 1997) |
- There is no routine place for the use of the mineralo-corticoid suppression adaptation of the sweat test.
|
B |
(Hodson et al., 1983)
|
| Responsibility for Testing and Training |
|
|
- Sweat collection must be performed by fully trained and experienced personnel:
- Training schedules should be fully documented.
- The procedure should be documented as an SOP.
- Appropriate revalidation procedures should be in place.
|
C |
|
- Sweat collection can be undertaken by a variety of health professionals.
|
C |
|
- Sweat analysis should be performed by qualified and experienced biomedical scientists or clinical scientists who are fully trained with regular validation:
- Training and validation schedules should be fully documented.
|
C |
|
- A consultant (or equivalent) clinical chemist should have responsibility for training, assessment of competence and revalidation for all staff undertaking sweat tests.
|
C |
|
- A minimum number of 50 sweat tests per annum should be performed in any one centre.
|
C |
|
- A minimum of 10 collection procedures should be performed per person per annum.
|
C |
|
- The responsibilities for sweat testing, both collection and analytical, should rest with a consultant (or equivalent) clinical chemist and should be clearly understood by all operators and users; a mechanism for reporting any concerns about performance should be in place and clearly understood.
|
C |
|
