About 1 in 2,500 babies born in the UK has cystic fibrosis (CF). This inherited condition can affect the digestion and lungs. Babies with CF may not gain weight well, and have frequent chest infections. The screening test measures immunoreactive trypsinogen in the blood spot. Screening means that babies with CF can be treated early with a high energy diet, medicines and physiotherapy. Although a child with CF may still become very ill, early treatment is thought to help them live longer, healthier lives. If babies are not screened for CF and the condition is suspected they can be tested later, usually with a sweat test.