Cystic fibrosis (CF) is most often diagnosed through newborn screening, which catches the majority of cases within the first few weeks of life. The process typically starts with a simple blood test and, if results are abnormal, moves through genetic testing and a confirmatory sweat test. Some people, however, aren’t diagnosed until adolescence or adulthood, when symptoms finally prompt a closer look.
Newborn Screening: The First Step
Every state in the U.S. screens newborns for cystic fibrosis as part of the standard heel-prick blood test done in the first day or two after birth. The test measures a substance called immunoreactive trypsinogen, or IRT, which is produced by the pancreas. Babies with CF tend to have elevated IRT levels in their blood.
A high IRT result alone doesn’t mean a baby has CF. In most cases, the same blood sample is then tested for changes in the CFTR gene, the gene responsible for cystic fibrosis. What happens next depends on how many gene changes are found. If testing reveals high IRT and two or more CFTR mutations, the baby very likely has CF and will be referred for confirmatory testing. If only one mutation is found alongside high IRT, the result is inconclusive, and the baby still needs follow-up, but a CF diagnosis isn’t certain. Some screening programs use a second IRT test drawn a couple of weeks later instead of immediate genetic testing, though the two-step approach with genetic analysis is more common.
Standard genetic panels check for a set of the most common CFTR mutations. California’s program, for example, uses a 40-mutation panel tailored to its population. When the panel detects only one mutation but suspicion remains high, some programs move to full gene sequencing to look for rarer mutations that a standard panel would miss. This layered approach improves detection rates without subjecting every newborn to extensive genetic analysis.
The Sweat Test: The Gold Standard
Regardless of what newborn screening or genetic testing shows, the sweat chloride test is the definitive diagnostic tool for cystic fibrosis. It measures how much chloride is in a person’s sweat, because the faulty CFTR protein in CF disrupts salt transport across cells, making sweat unusually salty.
During the test, a small area of skin (usually the forearm) is stimulated with a mild chemical and a tiny electrical current to trigger sweating. The sweat is collected for about 30 minutes and sent to a lab for analysis. The test is painless, though younger infants sometimes don’t produce enough sweat for an accurate reading, requiring a repeat attempt.
The results fall into three categories:
- Below 40 mmol/L: CF is unlikely.
- 40 to 60 mmol/L: Borderline. The test will need to be repeated, and additional genetic or functional testing may be necessary to rule out atypical forms of CF.
- Above 60 mmol/L: A CF diagnosis is confirmed.
Most babies flagged by newborn screening will have their sweat test done within the first month of life. A positive sweat test on two separate occasions, combined with either two identified CFTR mutations or clinical symptoms, establishes the diagnosis.
Genetic Testing Beyond Screening
Genetic testing plays a dual role in CF diagnosis. It confirms the condition and identifies which specific CFTR mutations a person carries, which matters because different mutations affect disease severity and treatment options. Over 2,000 CFTR mutations have been identified, though a relatively small number account for most cases.
When initial panels don’t find two mutations but clinical suspicion is strong, full CFTR gene sequencing can uncover rare variants. This is particularly useful in populations where less common mutations are more prevalent. Even with sequencing, a small number of people with clinical CF may have mutations that current tests can’t detect, which is why the sweat test remains essential as an independent confirmation.
Physical Signs That Trigger Testing
Some babies show signs of CF before screening results even come back. About 20% of people eventually diagnosed with CF are born with meconium ileus, a bowel obstruction caused by abnormally thick meconium (the dark, sticky first stool). When meconium ileus is identified, either after birth or suspected on a prenatal ultrasound showing echogenic (bright) bowel, genetic testing for CF is offered promptly. If a prenatal ultrasound raises concern, parents may be tested for their carrier status first. Two carrier parents means a 25% chance the baby has CF.
Other early signs that may prompt diagnostic testing in infants and young children include failure to gain weight despite adequate feeding, frequent greasy or bulky stools, recurrent lung infections, and a salty taste to the skin that parents sometimes notice when they kiss their baby.
Diagnosis in Older Children and Adults
Not everyone with CF is caught by newborn screening. Some people carry milder CFTR mutations that produce enough functional protein to delay obvious symptoms for years or even decades. These individuals may first come to medical attention with recurrent sinus infections, chronic bronchitis, unexplained bronchiectasis (permanent widening of the airways), recurring bouts of pancreatitis, or infertility in men caused by absence of the vas deferens.
When CF is suspected later in life, the diagnostic process is the same: sweat chloride testing and CFTR genetic analysis. The challenge is that adults with milder mutations sometimes have borderline sweat chloride values in the 40 to 60 range, making the diagnosis less straightforward. A person might also be found to have CFTR dysfunction that causes a single organ problem, like infertility or pancreatitis, without meeting the full diagnostic criteria for CF. This is classified as a CFTR-related disorder, a distinct category that still requires monitoring but carries a different prognosis than classic CF.
Testing for Borderline Cases
When sweat chloride results are borderline and genetic testing is inconclusive, specialized tests can measure how the CFTR protein is actually functioning in the body. One such test is nasal potential difference (NPD) measurement, which assesses how ions move across the lining of the nose. In CF, the abnormal CFTR protein creates a distinctive electrical pattern that differs from healthy tissue. The test involves placing a small electrode on the nasal lining and measuring voltage changes as different solutions are applied.
NPD testing isn’t widely available and is typically performed at specialized CF centers. It’s most useful when the standard combination of sweat testing and genetic analysis hasn’t produced a clear answer. Importantly, relying on baseline voltage alone isn’t enough. The full protocol, which includes measuring the tissue’s response to chloride-free solutions and specific medications that activate chloride channels, is needed to accurately distinguish CF from non-CF patterns.
Another functional test, intestinal current measurement, works on a similar principle using a small rectal biopsy sample. Both tests exist primarily for the subset of patients whose diagnosis remains uncertain after standard workup.
What a Diagnosis Sets in Motion
Once CF is confirmed, the next steps happen quickly. Babies and newly diagnosed individuals are typically referred to an accredited CF care center, where a multidisciplinary team coordinates treatment. Early baseline testing usually includes lung function measurements (when the child is old enough), cultures to check for specific bacteria, pancreatic function tests, and nutritional assessments.
Knowing the exact CFTR mutations is increasingly important because newer therapies called modulators target specific mutation types. The most widely prescribed modulator combination works for roughly 90% of people with CF who carry at least one copy of the most common mutation. Genetic results obtained during diagnosis directly determine eligibility for these treatments, which is one reason thorough genetic testing early on has lasting value.