Diagnosis

A number of methods can be used for diagnosis of patients, depending on requirements. Newborn screening can diagnose cystic fibrosis a few days after birth, but it is not always used. The majority of patients are diagnosed by an evaluation after symptoms begin to show. This diagnosis is usually the result of a sweat test.

Patients who wish to know if they are a carrier of CF before having children can use the carrier test. However, if a woman is already pregnant and there is a risk that the developing foetus could have CF an antenatal test can be conducted.
 

Overview of Diagnostic Methods:

Newborn Screening – Guthrie Test

This is a screening test which carried out on all babies in the UK a few days after they are born. It includes tests for sickle cell disease, phenylketonuria, congenital hypothyroidism, MCAD deficiency and cystic fibrosis.

Blood is taken from the babies’ heel and analysed, measuring immunoreactive trypsin assays on blood spots. 

Carrier Testing

This is a simple test involving the use of a mouthwash for mutation analysis. It should be carried out if there is a known family history of cystic fibrosis, or if someone’s partner is a carrier.

This test is able to look for 90% of mutations, so if someone is carrying a mutation in an unusual gene it may not be identified. Therefore even if this test shows negative results there is still a slight chance that the person could be a CF carrier.

Antenatal Testing

This test is carried out on the developing foetus if both parents are shown to be carriers of CF.  This is usually performed in approximately the 10th week of pregnancy using the chorionic villus sampling technique (CVS).

The CVS technique tests a small sample of chorionic villi (placental tissue) for the presence of a gene mutation associated with cystic fibrosis. The sample of placental tissue is collected either by using a thin needle passed through the stomach wall or by passing a tube up the cervix. 

Picture of foetus. Image courtesy of https://commons.wikimedia.org/wiki/File:Color20_weeks_pregnant.jpg under the terms of the GNU free documentation licence

Sweat Test

CF patients have higher salt levels than normal present in their sweat. This test uses a sweat producing chemical (e.g. pilocarpine) which is applied to the skin on the patient's arm/leg.

 An electrode is then used to provide a mild current which produces sweat, causing a warm sensation. This area is then covered with a piece of gauze to collect the sweat sample and is used for analysis. High salt levels indicate CF.  

X-ray of Lungs

A chest x-ray can reveal scarring and inflammation present in people with CF.

This X-ray (on right) shows a post-transplant lung on the left compared to an end stage cystic fibrosis lung on the right.

The difference between them demonstrates how a chest x-ray can be used to easily establish if a patient has cystic fibrosis.

Images of Guthrie test https://www.flickr.com/photos/rasulmcmahon/497094272/ and chest x-ray https://www.flickr.com/photos/heitkamp/448873010/ courtesy of Flickr under the creative commons licence