Can cystic fibrosis be missed or misdiagnosed? 

While the United Kingdom has a robust national screening programme designed to identify cystic fibrosis at birth, it is still possible for the condition to be missed or misdiagnosed. This most commonly occurs in individuals born before universal screening was implemented, or in those with “atypical” genetic mutations that result in milder symptoms. Because cystic fibrosis can mimic other common conditions, such as asthma, chronic bronchitis, or primary ciliary dyskinesia, patients may spend years being treated for alternative respiratory or digestive issues before the underlying genetic cause is identified. Understanding the factors that lead to a delayed diagnosis is essential for ensuring that all patients receive the specialist care they require. 

What We’ll Discuss in This Article 

• Why some individuals were not captured by newborn screening. 

• Common conditions that cystic fibrosis is often mistaken for. 

• The challenge of diagnosing “atypical” or “residual function” cases. 

• How “borderline” test results can lead to clinical uncertainty. 

• The role of rare genetic mutations in diagnostic delays. 

• When to seek a specialist review for persistent, unexplained symptoms. 

Limitations of newborn screening 

The primary reason cystic fibrosis is rarely missed in children today is the newborn blood spot test. However, this system is not infallible. According to the NHS, the screening programme identifies about 95 percent of cases, meaning a very small number of children with rare mutations may not be detected at birth. Additionally, universal screening was only fully rolled out across the UK in 2007. This means a significant portion of the adult population was never screened as infants. Unless these individuals showed severe “classic” symptoms in childhood, such as extreme malnutrition or frequent pneumonia, their condition may have remained undiagnosed for decades. 

Common misdiagnoses in respiratory health 

Cystic fibrosis is frequently misdiagnosed as other chronic respiratory conditions because the symptoms such as coughing, wheezing, and shortness of breath are very similar. Many adults eventually diagnosed with cystic fibrosis were previously told they had severe asthma that did not respond well to standard inhalers. Others may have been diagnosed with recurring bronchitis or bronchiectasis (widening of the airways) without the underlying cause being investigated. NICE guidance suggests that clinicians should consider cystic fibrosis in any patient with unexplained bronchiectasis or chronic sinus disease with nasal polyps, as these are often the primary presenting symptoms in late-onset cases. 

The challenge of “atypical” cystic fibrosis 

Diagnosis is particularly difficult when a person has “atypical” cystic fibrosis. This occurs in individuals with “residual function” mutations, where the CFTR protein works just well enough to prevent the severe, early-onset symptoms typically associated with the condition. These patients may not have the greasy stools or severe malnutrition seen in classic cases because their pancreas continues to function correctly. Instead, they may only experience symptoms in one organ system, such as chronic sinusitis or male infertility. Because they do not fit the “classic” profile of a cystic fibrosis patient, both they and their doctors may not consider genetic testing as a first-line investigation. 

Inconclusive and borderline test results 

Another factor that can lead to misdiagnosis or a “missed” diagnosis is the occurrence of borderline test results. The sweat test is the gold standard for diagnosis, but some people with milder mutations have salt levels that fall into a “grey area” (between 30 and 59 mmol/L). The NHS states that in these cases, the sweat test alone may not be enough to confirm or rule out the condition. If a clinician relies solely on a single sweat test without performing detailed genetic sequencing, they may incorrectly tell a patient they do not have the condition, only for symptoms to progress over the following years. 

Rare mutations and genetic testing limits 

Standard genetic testing in the UK looks for a panel of the most common mutations found in the British population. While this captures the vast majority of cases, there are over 2,000 known mutations in the CFTR gene. If an individual has an extremely rare mutation not included in the standard panel, their genetic test might come back “negative” or only show one mutation (carrier status) even if they have the disease. In these complex cases, specialist centres must perform “full gene sequencing” to look at every part of the gene. Failure to pursue this advanced testing in the face of strong clinical symptoms can lead to a missed diagnosis. 

Symptoms that should trigger a review 

In the UK, medical professionals are encouraged to look for “red flag” symptoms that might suggest an undiagnosed case of cystic fibrosis, particularly in adults. These include: 

• Unexplained, persistent bronchiectasis seen on a CT scan. 

• Recurring bouts of pancreatitis without a clear cause (like alcohol or gallstones). 

• Chronic sinusitis that persists despite multiple surgeries or treatments. 

• Male infertility caused by the absence of the vas deferens (CBAVD). 

• A persistent, productive cough that has been present since childhood. 

• Unusual salt depletion or heat exhaustion during exercise or hot weather. 

Condition	Often Misdiagnosed as…	Key Differentiating Factor
Cystic Fibrosis	Asthma	CF produces thick mucus and does not fully clear with inhalers.
Cystic Fibrosis	Celiac Disease	CF involves respiratory issues and high sweat salt levels.
Cystic Fibrosis	Recurrent Bronchitis	CF is a lifelong genetic fault, not just recurring infections.
Cystic Fibrosis	Chronic Sinusitis	CF often involves nasal polyps and a genetic underlying cause.

Conclusion 

Cystic fibrosis can be missed or misdiagnosed, particularly in adults born before 2007 or those with milder genetic mutations that do not present with “classic” symptoms. The similarity between cystic fibrosis symptoms and common conditions like asthma often leads to diagnostic delays. However, with the use of detailed genetic sequencing and a high index of clinical suspicion for symptoms like bronchiectasis or male infertility, specialist centres in the UK can correctly identify these cases. A correct diagnosis is vital at any age to ensure access to treatments that can slow the progression of organ damage. 

If you experience severe, sudden, or worsening symptoms, call 999 immediately. 

Authority Snapshot (E-E-A-T Block) 

This article examines the clinical challenges regarding the misdiagnosis of cystic fibrosis, aligning with the standards set by the NHS and NICE. The content is authored by a medical content team and reviewed by Dr. Rebecca Fernandez, a UK-trained physician with experience in internal medicine, cardiology, and emergency care. This information is intended to help patients and families understand why diagnostic delays occur and the importance of seeking specialist assessment for persistent, unexplained symptoms.