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Physicians Reference Laboratory                           Saint Luke’s Northland Hospital                            Providence Medical Center Mary’s Hospital of Blue Springs                   Saint Joseph Health Center                                    Mayo Medical Laboratories

Vol 7  Issue 3                                                                              October 2003

Cystic Fibrosis

Dr.Fred Plapp , Dr.Marilyn Hamilton and Uttam Garg, PhD.

Cystic fibrosis is the most common life-limiting autosomal recessive genetic disorder in Caucasians, with an incidence of 1 in 3300 European Caucasian newborns and 1 in 30,000 for the entire US population. 

Cystic fibrosis is caused by a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which codes for a protein that serves as a chloride ion channel in epithelial cell membranes.  Mutations that result in defective CFTR function cause cells to be impermeable to chloride conductance, leading to increased chloride and sodium concentration in sweat and highly viscous mucus. The latter is responsible for most of the clinical and pathological effects of cystic fibrosis, including respiratory and digestive problems.

Prenatal Screening

Discovery of these CFTR mutations has facilitated genetic screening for carrier status. In March 2001, the American College of Medical Genetics (ACMG) and the American College of Obstetrics and Gynecology (ACOG) recommended that all Caucasian couples who are pregnant or are considering pregnancy be offered carrier screening for cystic fibrosis. They also recommended that non-Caucasian couples be made aware of the availability of screening. The rationale for these recommendations is that carrier detection allows for earlier identification of at risk couples who would then have the ability to exercise their reproductive options, including prenatal diagnosis and earlier treatment of affected children.   

More than 900 different mutations have been detected, but most are very rare.  Only a few dozen have an allele frequency of greater than 1 in 1000 in the general population. Studies in the general Caucasian population revealed that a panel of 15 to 20 mutations detected more than 80% of carriers.  Analysis of only 7 mutations detected 97% of carriers in the Ashkenazi Jewish community.  Because it is not practical or cost effective to test for all 900 mutations, ACOG and ACMG have recommended that a pan-ethnic core panel of the 25 most common mutations should be used for carrier screening. 

Because this panel is limited, some carriers will not be detected. The detection rate of this panel varies with ethnicity.  The following table lists the pre-test carrier rate, disease prevalence, screening detection rate of the panel and the remaining carrier risk for an individual who has a negative result.

Prevalence and Estimated Carrier Risk

                                                Pre- Test          Disease            Screening          Carrier Risk After

Ethnic Group Carrier Rate Prevalence Detection Rate Negative Test

European Caucasian                  1:29                  1:3,300                    80%                       1:140

Ashkenazi Jewish                      1:29                  1:3,300                    97%                       1:930

Hispanic American                     1:46                  1:8500                     57%                       1:105

African American                       1:65                  1:15,300                   69%                       1:207

Asian American                         1:90                  1:32,100                   30%                       1:125

Screening may be performed either before pregnancy or during the first trimester. Preconception screening is preferable.  Screening of both partners can be accomplished either concurrently or sequentially.  Concurrent screening of both partners is recommended for Caucasians and Ashkenazi Jews.  This provides a more rapid and precise estimate of carrier status and the risk of cystic fibrosis in their offspring.  Sequential screening is the screening of one partner followed by screening of the second partner only in the event of a positive result in the first partner.  Since most individuals test negative, this strategy is less expensive but takes longer.  Also, since the screening tests are not 100% sensitive, there is a higher residual risk when only one partner is tested and found to be negative and the carrier status of the other partner remains unknown. 

Saint Luke’s Regional Laboratories began performing cystic fibrosis screening on November 26, 2002. Current turnaround time is one week. Request forms and patient brochures are available from Saint Luke’s Regional Laboratories. Request forms must include the patient’s ethnic background and their family history of cystic fibrosis in order for the laboratory to estimate risk accurately. Specimen requirement is one lavender top (EDTA) tube of blood.

Diagnosis

Cystic fibrosis is usually diagnosed in infants. Meconium ileus is common in the newborn. Respiratory insufficiency with bronchiectasis and malabsorption with failure to gain weight and bulky stools are the most common symptoms. Sometimes a care giver will report that the baby tastes salty when kissed.

The Cystic Fibrosis Foundation (www.cff.org) sets the standards for the diagnosis and care of patients with Cystic Fibrosis.  Children’s Mercy Hospital is accredited by the Cystic Fibrosis Foundation and that accreditation includes inspection of the laboratory for its testing procedures. The sweat test has been the “gold standard” for diagnosis for 40 years and remains so today. The sweat test determines the amount of chloride in sweat.  Sweating is induced on the forearm by pilocarpine iontophoresis and the sweat is collected on gauze or filter paper or in microbore tubing for analysis. A minimum amount of sweat must be collected for a valid test. Other methods are available but are not as reliable and are not sanctioned by the Cystic Fibrosis Foundation. Tests need to be interpreted in the context of the family history and clinical presentation. Values 0-40 mmol/L are considered negative but very rarely can be found in patients with genetically proven CF.  Borderline values (41-60 mmol/L) require additional workup. Values over 60 mmol/L are consistent with CF. There are other conditions that can increase sweat chloride.  The Cystic Fibrosis Foundation recommends repeating all positives on a separate occasion.

It takes about one hour to complete the collection of sweat from the patient and requires an appointment which can be made by phone (816-234-1530 for Main Campus or 913-696-8210 for Children’s Mercy South). The collection process is difficult. . In order to minimize technical failures only specially trained people with frequent experience perform the collections.

A small fraction of CF patients do not have diagnostic sweat chloride results, borderline results, or may not sweat enough to do the sweat chloride test. In these cases there are two choices. The sweat test can be repeated. Alternatively, a molecular genetic test can be performed. Children’s Mercy Hospital sends genetic testing to Genzyme Laboratories where the test includes 86 mutations. Please call the lab 816-931-8080 ahead of time for details. Approximately 3 cc in a purple top tube is required.  There is a form that needs to be filled out providing the lab with clinical information. There are a few patients who have positive sweat tests and are affected (have CF) and have only one or even no identifiable genetic mutation. The detection rate of this molecular genetic test is 92.6% in Caucasian patients.

Patients with CF have frequent respiratory infections. The Microbiology Laboratory at Children’s Mercy Hospital is inspected by the Cystic Fibrosis Foundation and accredited for the special procedures it uses to diagnosis respiratory infections in patients with CF.