Cystic Fibrosis Newborn Screening in Austria Using PAP and the Numeric Product of PAP and IRT Concentrations as Second-Tier Parameters

Abstract

In Austria, newborns have been screened for cystic fibrosis (CF) by analyzing immunoreactive trypsinogen (IRT) from dried blood spots (DBS)s for nearly 20 years. Recently, pancreatitis-associated protein (PAP) analysis was introduced as a second-tier test with the aim of reducing recalls for second DBS cards while keeping sensitivity high. For 28 months, when IRT was elevated (65-130 ng/mL), PAP was measured from the first DBS (n = 198,927) with a two-step cut-off applied. For the last 12 months of the observation period (n = 85,421), an additional IRT×PAP cut-off was introduced. If PAP or IRT×PAP were above cut-off, a second card was analyzed for IRT and in case of elevated values identified as screen-positive. Above 130 ng/mL IRT in the first DBS, newborns were classified as screen-positive. IRT analysis of first DBS resulted in 1961 (1%) tests for PAP. In the first 16 months, 26 of 93 screen-positive were confirmed to have CF. Two false-negatives have been reported (sensitivity = 92.8%). Importantly, less than 30% of families compared to the previous IRT-IRT screening scheme had to be contacted causing distress. Adding IRT×PAP caused a marginally increased number of second cards and sweat tests to be requested during this period (15 and 3, respectively) compared to the initial IRT-PAP scheme. One case of confirmed CF was found due to IRT×PAP, demonstrating an increase in sensitivity. Thus, the relatively simple and economical algorithm presented here performs effectively and may be a useful model for inclusion of CF into NBS panels or modification of existing schemes.

Keywords: IRT-PAP; IRT×PAP; false-positives; neonatal screening; recalls.

Figure 1

CF Screening schemes in Austria. Algorithms for the first period (A) and the second period (B) are depicted. Bold numbers give numbers of cards within indicated categories. The number in brackets is the number of confirmed positives within these categories. IRT and PAP cut-off concentrations are given in ng/mL, IRT × PAP in ng²/mL².

Figure 2

PAP vs. IRT values from first DBSs. Values of samples with IRT between 65 and 130 ng/mL eligible for PAP analysis according to the screening scheme for (A) first period and (B) second period are plotted. Solid lines indicate used cut-off values for request of second card; dashed lines give comparison to alternative strategies (see main text). Black data points indicate confirmed CF cases.