What laboratory test is considered definitive in diagnosing cystic fibrosis?

The sweat chloride test is considered definitive for diagnosing cystic fibrosis because it measures the concentration of chloride in sweat. In individuals with cystic fibrosis, mutations in the CFTR gene prevent the proper functioning of the CFTR protein, which regulates chloride channels. As a result, patients with cystic fibrosis often have a significantly higher concentration of chloride in their sweat compared to individuals without the condition.

For a diagnosis to be confirmed, the test typically shows a sweat chloride level above a certain threshold, usually around 60 mmol/L or greater. This test is non-invasive and has been the gold standard in diagnosing cystic fibrosis due to its specificity and sensitivity.

While other tests can provide supportive information for diagnosing cystic fibrosis, they do not have the same level of diagnostic reliability. Complete blood counts, chest X-rays, and pulse oximetry are important in evaluating a patient's general health and respiratory function but do not provide definitive evidence for or against cystic fibrosis. Thus, the sweat chloride test stands out as the crucial test in the diagnostic process for cystic fibrosis.