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Diagnosis of Cystic Fibrosis

A diagnosis of cystic fibrosis is established when a patient with suggestive symptoms or an abnormal neonatal screening test has two measures of sweat chloride above 60 mEq/L or two mutations detected in genetic testing.

  • Neonatal screening (heel prick test): Neonatal screening consists of collecting a few drops of blood from the baby's heel in the first week of life. A neonatal screening test is not definite, that is, it does not determine whether the baby has cystic fibrosis or not. It can, however, indicate that additional tests are needed to confirm whether the patient has the disease. Doing a heel prick test is essential because it allows the diagnosis of cystic fibrosis before the first symptoms appear, which greatly improves the prognosis of patients. In addition, early diagnosis allows genetic counseling for parents and family members regarding future pregnancies. A heel prick test also scans several other potentially serious diseases. Learn more about neonatal screening (text in Portuguese) at
  • A sweat test is done only when cystic fibrosis is suspected, and the approach consists of sweat induction and collection. The test measures the concentration of chloride in sweat, and levels above the reference range indicate the presence of the disease (60 mEq/L or higher, depending on the technique employed).
  • Genetic testing: This test is an option for diagnosing people with suspected cystic fibrosis who have intermediate (between 40 and 60 mEq/L) or even normal levels of chloride in sweat. It is also useful for determining the prognosis of a patient, providing counseling to the family, and establishing possible treatments with new medications that "correct" the CFTR protein function.

When the mother of a baby has cystic fibrosis or when there are cases in the family, diagnostic investigation is recommended using a sweat test or a genetic test. A genetic test also allows, in some cases, prenatal diagnosis, that is, before the baby is born.

  • Prenatal diagnosis: A prenatal diagnostic test for evaluating whether the fetus has cystic fibrosis is done only if previous tests have confirmed that both the mother and the father of the baby carry the gene affected by the disease (CFTR). This is because the baby will only have cystic fibrosis if the affected genes are inherited from both the mother and the father. The prenatal test consists of collecting a sample of amniotic fluid from the uterus to determine the diagnosis of cystic fibrosis. This test is usually done between weeks 15 and 18 of pregnancy.

Even when the parents do not know if they carry the affected gene, cystic fibrosis screening should ideally be performed in the first days of life using the heel prick test (neonatal screening). Although this is a mandatory test in Brazil, access remains limited throughout the country. For this reason, the diagnosis is sometimes delayed, occurring after the first symptoms appear. This delay in the diagnosis of cystic fibrosis may compromise the overall health and prognosis of the patient