From the Guidelines
A chloride spot test result of less than 20 mmol/L indicates that the sweat chloride concentration is normal, suggesting the individual likely does not have cystic fibrosis. This test measures the amount of chloride in sweat and is a key diagnostic tool for cystic fibrosis, a genetic disorder affecting the exocrine glands. According to the British Thoracic Society guideline for bronchiectasis in adults 1, a negative sweat test has <30 mmol/L chloride, which supports the notion that a result of less than 20 mmol/L is within the normal range. In healthy individuals, sweat typically contains low levels of chloride, and people with cystic fibrosis have a defect in the CFTR protein that regulates chloride transport across cell membranes, resulting in abnormally high chloride levels in sweat, typically above 60 mmol/L. Values between 30-60 mmol/L are considered borderline and may require additional testing. If a patient receives this normal result but still has symptoms suggestive of cystic fibrosis, further diagnostic evaluation including genetic testing for CFTR mutations might be warranted, as some atypical forms of cystic fibrosis can occasionally present with normal or borderline sweat chloride levels. Key points to consider include:
- The chloride spot test is a diagnostic tool for cystic fibrosis
- A result of less than 20 mmol/L is considered normal
- Further testing may be necessary if symptoms persist despite a normal result
- Genetic testing for CFTR mutations can provide additional diagnostic information 1.
From the Research
Chloride Spot Test Result Interpretation
A chloride spot test result of less than 20 mmol/L indicates a low level of chloride in sweat. This result can be interpreted as follows:
- It is unlikely to be indicative of cystic fibrosis (CF), as most patients with CF have sweat chloride levels greater than 60 mmol/L 2, 3, 4.
- However, it is essential to note that a normal sweat chloride test does not entirely rule out CF, especially in cases where there is a strong clinical suspicion or family history of the disease 5.
- Malnutrition can cause elevated sweat chloride levels, but in this case, the level is low, so malnutrition is unlikely to be a contributing factor 6.
- The result may be considered normal, as seen in the study where the median sweat chloride value in patients without CF was 11 mmol/L 2.
Key Points to Consider
- Sweat chloride levels can vary, and a single test result should be interpreted in the context of clinical presentation and other diagnostic criteria.
- Borderline or normal sweat chloride values may require further investigation, including genetic testing, to confirm or rule out CF 4, 5.
- The chloride spot test is just one aspect of diagnosing CF, and other factors, such as clinical symptoms, family history, and genetic mutations, should also be considered 2, 3, 4, 5.