Multiple Positive Sweat Chloride Tests in an Infant Asymptomatic for Cystic Fibrosis

Steven W. Cotten, PhD; Laura M. Bender, PhD; Monte S. Willis, MD, PhD


Lab Med. 2012;43(2):1-5. 

In This Article

Abstract and Introduction


Clinical History
Patient: 1-month-old male.
Chief Complaint: Multiple positive sweat chloride tests in the absence of signs or symptoms suggestive of cystic fibrosis (CF).
History of Present Illness: The patient was referred for sweat chloride testing after a positive newborn screen for immunoreactive trypsinogen (IRT). Immunoreactive trypsinogen is suggestive of CF when the value is within the top 5% of a period-specific floating cutoff value[1] (Figure 1). The patient's sweat chloride levels were elevated (>60 mmol/L). Preliminary screening for CF transmembrane conductance regulator (CFTR) mutations using a 40 mutation screen identified 1 mutation in intron 11 (1585 -1G>A, legacy name 1717 -1G>A). Subsequent full gene CFTR sequencing revealed a second CFTR mutation in intron 22.
Past Medical History: The infant was full term, weighing 8 lbs 6 oz at birth, and experienced an uncomplicated delivery. The infant was discharged following a standard 2-day stay in the newborn nursery.
Physical Exam: A temperature of 36.4°C (axillary), blood pressure of 120/80 mm Hg, heart rate of 148, and respirations of 40 per minute were measured for the patient. The patient's O2 saturation was 100% on room air. The patient's length was 61.2 cm (90th percentile), weight 6.27 kg (95th percentile), and head circumference was 39.7 cm (50th percentile). His weight-to-length ratio was at the 50th percentile. The patient appeared healthy with no labored respiration and no apparent digestive issues.
Drug History: None
Family History: No family history of chronic diseases.
Principal Laboratory Findings: The patient underwent sweat chloride testing on sweat collected by pilocarpine iontophoresis from both arms (Table 1). These tests were repeated several weeks later yielding similar results (not shown).


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