Guided case: intermediate becomes confirmatory evidence

A term infant, birth weight 3.4 kg, is in an IRT/DNA state. The first dried-blood-spot IRT is above the floating cutoff, and the panel detects F508del with no second variant. On day 24, the infant is above 2 kg, at least 36 weeks corrected gestational age, and older than 10 days, so confirmatory sweat testing can proceed. These gates support collection timing but do not interpret the eventual chloride result.

Simultaneous bilateral Macroduct-type collections run for 30 minutes. The right arm yields 30 microliters and gives 48 mmol/L, which is intermediate under the CFF 2017 category of 30 to 59 mmol/L. The left arm yields 9 microliters, below the 15-microliter minimum, and is reported QNS rather than analyzed or pooled. Low- and high-level QC are within the laboratory acceptance limits for this run.

Two weeks later, separately repeated adequate collections yield 62 and 65 mmol/L. Full CFTR sequencing identifies a second CF-causing variant in trans with F508del. With positive NBS, repeat elevated sweat chloride, and compatible molecular evidence, the complete clinical picture meets CFF diagnostic criteria, while the laboratory report states method, units, adequacy, result, and interpretive category. The intermediate first result triggers repeat testing and further evidence, not a premature conclusion.