False negative CF screening test

Question

Can you explain how a negative CF screening test (at birth) can be a false negative ?
Are such cases common? How can this happen?
My son had this test done at birth, it was negative. After four months, and after many visits to the hospital, it turns out that he is suffering from cystic fibrosis.

Answer

Hello,
Screening tests are not diagnosis tests. They:
- apply to a population of individuals who do not have specific symptoms;
- seek to distinguish people who probably have the disease from people who probably have not;
- are not intended to diagnose the illness which must be confirmed or eliminated by other examinations: a medical examination and other laboratory tests.
Ideally, I agree, it would be to have a perfect test, both 100% sensitive, say able to detect all cases of CF (no false negative) and 100% specific, say, never positive for a safe baby (no false positive). If it was so, the screeing test would be both a screening and a diagnosis test.
Systematic CF neonatal screening identifies more than 95% of babies with CF. This test, like most screening tests, even done correctly, knows a certain percentage of false negatives (5%). In other words, even if the screening is positive, diagnosis of CF :
- should be suspected in patients demonstrating signs such as poor weight gain, recurrent respiratory infections or chronic diarrhea,
- then confirmed or excluded in an accreditated CF Center by a medical examination, an appropriate sweat test and, if it is positive or intermediate, genetic analysis for CF gene mutation identification.
You will find more information by clicking on the following link (French):
www.medecine.ups-tlse.fr/DCEM2/module1/sous_module1/011_depistage_CA_SA.pdf

Sincerly
Gilles RAULT, MD
Roscoff CF Center, France

29.04.2014