Table 1.

CF Screening False Negatives and the Cause of Delayed Diagnosis after IRT/DNA Analyses

Year	Mutation 1	Mutation 2	IRTa	Age at Diagnosis	Sweat Test Results (mmol/L)	Cause of False Negative
1995	p. R553X	Unknown	53 (56)	4 months old	94	IRT below the cut-off
1996	p. R553X	p. R1161X	64 (56)	6 weeks old	109	F508del alone as the 2nd tier
1997	p. R347P	Unknown	82 (56)	7 weeks old	108	F508del alone as the 2nd tier
2000	3007delG	Unknown	99 (64)	9 months old	110	Rare mutations
2001	Unknown	Unknown	44 (66)	7 years old	59	IRT below the cut-off
2002	p. G551D	p. Q1291Hc	53 (64)	4 years old	77	IRT below the cut-off
2003	F508del	F508del	45 (51)	1 year old	121	IRT below the cut-off
2004	p. R170Hc	Unknown	13 (62)	3 years old	66	IRT below the cut-off

^aIRT was reported as ng/mL. The value reported in the parenthesis was the IRT cutoff value for that testing date. In 1999, Wisconsin changed from a fixed cutoff value for IRT to a floating cutoff calculated on a daily basis as described elsewhere [17].

^bNote that all sweat chloride values were diagnostic of CF [20] and that 6 of the 8 patients were identified after 2 months of age.

^cCFTR mutation associated with pancreatic sufficiency, while all other alleles list are associated with pancreatic insufficiency