False negative results on newborn screening for cystic fibrosis

Over a 5 year period in Newcastle, 18 new cases of cystic fibrosis (CF) were diagnosed in children who had been screened in the newborn period. In six of these children, the screening programme failed. Four of these children had a normal screen and an additional two had elevated immunoreactive trypsin (IRT), but there were problems with the notification procedure. Three of the children missed by the screening process had a significantly delayed diagnosis; in all three cases the diagnosis of CF was suspected clinically, but a sweat test was delayed because of false reassurance from the fact that the child had been screened for CF. In a fourth case, multiple elevated sweat electrolyte levels were obtained, but the diagnosis of CF was considered to be in doubt because of the normal IRT assay. A sweat test should be performed on any child in whom there is clinical suspicion of CF.     

Failure to thrive: the earliest feature of cystic fibrosis in infants diagnosed by neonatal screening

The benefits of early treatment of nutritional and respiratory problems in the CF infant and of genetic counselling for the parents are widely recognized. However, clinical diagnosis of CF is often delayed despite early onset of symptoms and the usefulness of neonatal population screening as a preventive measure is still under debate. This study analyses the clinical history of CF patients diagnosed exclusively on the basis of positive neonatal screening tests with the aim of identifying the earliest markers of the disease. We studied 103 CF infants bom in north-east Italy, diagnosed following neonatal screening: assay of immunoreactive trypsin (IRT) from a heel-prick blood sample followed by a measurement of meconium lactase in cases with raised IRT. Diagnosis was confirmed by sweat test at an average age of 39 days. Eighty-one patients (79%) had symptoms strongly suggestive of CF at diagnosis, and signs and/or symptoms of pancreatic insufficiency were present in 16 of the remaining 22 cases. The most frequent symptom was growth failure (69% of infants) and of these. 44% weighed the same as at birth or less. Pancreatic insufficiency was confirmed by the low level of faecal chymotrypsin found in 85% of cases. IRT was elevated in all cases. CF had not been suspected in any symptomatic infant, although most of the infants had been monitored by a paediatrician. In conclusion, most infants with CF diagnosed by neonatal screening are already symptomatic in the first six weeks of life and the most frequent symptom is failure to thrive; pancreatic insufficiency was already present in most cases. In areas without CF neonatal screening programs, the disease should be excluded by differential diagnosis in all cases with growth failure notwithstanding adequate caloric intake in the first months of life. The high sensitivity, low cost and simple execution of IRT and fecal chymotrypsin tests make them an ideal first step in suspect cases before proceeding to the sweat test, often performed late because of limited availability.     

Pancreatic function and extended mutation analysis in DeltaF508 heterozygous infants with an elevated immunoreactive trypsinogen but normal sweat electrolyte levels

BACKGROUND: Newborn screening for cystic fibrosis (CF) with immunoreactive trypsinogen (IRT) and DeltaF508 analysis followed by sweat testing misses some infants with CF and detects more DeltaF508 carriers than expected. Some of the apparent DeltaF508 carriers may be DeltaF508 compound heterozygotes with normal sweat electrolyte levels. METHODS: Infants identified by newborn screening with an elevated IRT level, one DeltaF508 allele, and a sweat chloride level <60 mmol/L underwent CF mutation analysis, pancreatic stimulation testing, and repeat IRT analysis followed by clinical review and repeat sweat test at 12 months. RESULTS: Over a 24-month period we identified 122 DeltaF508 heterozygotes and recruited 57; 4 had borderline sweat chloride levels (40 to 60 mmol/L), 5 (8.8%, 95% CI 1.4, 16.2) had a second CF mutation (R117H), and 11 (20%, 95% CI 10, 30) had the intron 8 5T allele. Three had clinical CF at 12 months (initial sweat chloride levels: 53, 51, and 32 mmol/L). Pancreatic electrolyte secretion in the subjects with a borderline sweat chloride level was similar to that in patients with known CF. CONCLUSION: The excess of DeltaF508 heterozygotes detected by IRT/DNA screening is associated with the presence of a second mutation or the 5T allele in some infants. Screened infants with borderline sweat chloride levels almost certainly have CF, but long-term follow-up of the infants with the genotype DeltaF508/R117H and DeltaF508/5T is required to determine their outcome. In the meantime, newborn screening should be confined to severe mutations associated with classic CF.     

Clinical complications in children with false-negative results in cystic fibrosis newborn screening

ObjectiveTo present signs and symptoms and clinical course in cystic fibrosis patients with false-negative newborn screening (CF NBS).Materials and methodsAll children presented in this paper were covered by CF NBS. The group of 1.869.246 newborns was screened in the Institute of Mother and Child in Warsaw within a period of 01.01.1999 – 31.05.2019. Screening protocols evolved over time from IRT/IRT to IRT/DNA/EGA.ResultsThe authors identified 11 patients with false-negative NBS, in whom CF was diagnosed based on clinical symptoms or the examination of siblings with positive CF NBS. In the study group, the diagnosis was made significantly later in comparison to positive CF NBS patients ranging from 2 months to 15 years of age. CF NBS strategy does not significantly affect the sensitivity of the screening.ConclusionIn the presence of clinical symptoms, additional diagnostics must be implemented, in spite of the negative screening results. At first, the sweat test should be conducted, followed by a DNA analysis of the most common mutations in the given population. The diagnostic process requires searching for CFTR mutations not typically associated with a high chloride concentration in sweat. Repetition of sweat chloride concentration enables the diagnosis in children whose initial chloride values in sweat are borderline, and no CF-causing mutations are detected. In strong clinical indications, the extension of DNA analysis (EGA) is recommended in order to identify rare CF variants. In children with meconium ileus, genetic analysis is mandatory.     

Experience with neonatal screening for cystic fibrosis in New Zealand using measurement of immunoreactive trypsinogen

Abstract Neonatal cystic fibrosis (CF) screening has been performed in New Zealand for a total of 7 years. This study reports the experience with this procedure in New Zealand over a 4 year period and compares it with 2 years when diagnoses of CF were suggested by clinical features only. A total of 72 infants were confirmed as having CF during 4 years of screening. Twenty-eight infants were found to have CF during 2 years in which screening was not performed. There were 29 false positive diagnoses during the screening years and six false negative diagnoses. Three of the false negative diagnoses occurred because of laboratory error, but three occurred because either the first or second measurement of immunoreactive trypsinogen (IRT) was normal. Faecal chymotrypsin was measured in samples from 434 infants at the time of the second IRT and assisted with the diagnosis for one infant which might otherwise have been missed. Only 42.5% of infants were asymptomatic at the time of the confirmatory sweat test. Significant morbidity and mortality was associated with meconium ileus which occurred in 24% of infants with CF. Improved ascertainment of cases of CF has occurred since screening began. Further follow-up is required to determine other benefits of newborn screening.     

Experience with neonatal screening for cystic fibrosis in New Zealand using measurement of immunoreactive trypsinogen

Neonatal cystic fibrosis (CF) screening has been performed in New Zealand for a total of 7 years. This study reports the experience with this procedure in New Zealand over a 4 year period and compares it with 2 years when diagnoses of CF were suggested by clinical features only. A total of 72 infants were confirmed as having CF during 4 years of screening. Twenty-eight infants were found to have CF during 2 years in which screening was not performed. There were 29 false positive diagnoses during the screening years and six false negative diagnoses. Three of the false negative diagnoses occurred because of laboratory error, but three occurred because either the first or second measurement of immunoreactive trypsinogen (IRT) was normal. Faecal chymotrypsin was measured in samples from 434 infants at the time of the second IRT and assisted with the diagnosis for one infant which might otherwise have been missed. Only 42.5% of infants were asymptomatic at the time of the confirmatory sweat test. Significant morbidity and mortality was associated with meconium ileus which occurred in 24% of infants with CF. Improved ascertainment of cases of CF has occurred since screening began. Further follow-up is required to determine other benefits of newborn screening.     

Negative sweat test in hypertrypsinaemic infants with cystic fibrosis carrying rare <Emphasis Type="Italic">CFTR</Emphasis> mutations

Persistent hypertrypsinaemia in newborn screening for cystic fibrosis (CF) recognises subjects at high risk to be affected. Diagnosis is confirmed by a positive sweat test and/or by the presence of two mutations in the cystic fibrosis transmembrane regulator gene. The aim of the present study was to evaluate the occurrence of a negative sweat test (chloride < 60 mmol/l) during the first months of life, in hypertrypsinaemic infants, which would lead to a delayed diagnosis. We reviewed clinical charts of CF patients born between January 1993 and September 1998, when the neonatal screening programme consisted of an immunoreactive trypsinogen (IRT)/DNA (F508del) + IRT strategy. Laboratory and clinical data were collected for patients diagnosed after 12 months of life. Out of 446,492 newborns, 104 CF patients were diagnosed giving an overall incidence of 1:4293. Of these, six had a blood IRT level above the cut off value (99th percentile) and a negative sweat test in the first trimester of life. At a mean age of 3.5years, the patients were again referred to our CF Centre for re-evaluation in order to confirm or exclude the disorder. Molecular analysis identified the following genotypes: F508del/A309D, F508del/3849 + 10kbC-->T, F508del/R117H (in two patients), R117H/ L997F, and F508del/R117L. CONCLUSION: Infants with cystic fibrosis bearing a spectrum of mild cystic fibrosis transmembrane regulator gene mutations may present as hypertrypsinaemic newborns with a sweat chloride within the normal range. Reference values for normal sweat test during the first months of life should be revised. A wide molecular genetic analysis is recommended for newborns presenting persistent hypertrypsinaemia and a sweat test result > 30 mmol/l in order to diagnose atypical forms of the disease.     

Cystic fibrosis screening in neonates — measurement of immunoreactive trypsin and direct genotype analysis for ΔF508 mutation

This study investigated the clinical usefulness of screening for cystic fibrosis (CF) in 19992 newborns, over 39 months, in an Austrian population. Immunoreactive serum trypsin (IRT) determination was followed by sweat chloride analysis (sweat test) to establish diagnosis. In a retrospective analysis covering 6 months of the study period, individuals who were considered to be at risk after IRT estimation (n = 22) were analysed for F508 mutation, using a new method of DNA extraction from the initial dried blood specimens. A total of 119 infants (0.6%) had values greater than 750mg trypsin/ml whole blood. In 88 babies sweat tests were performed, leading to the diagnosis of CF in 11 cases. One patient was not initially identified by screening but was later discovered due to his clinical status. Three infants were noted to carry the F508 mutation (1 homozygous, 2 heterozygous). Two of these babies already had CF. The second heterozygote was a carrier. A highly efficient three tier screening strategy is presented in which IRT estimation, determination of F508 status and sweat chloride testing could lead to a high sensitivity analysis of this population.     

Evaluation of 47,213 infants in neonatal screening for cystic fibrosis, using pancreatitis-associated protein and immunoreactive trypsinogen assays]

OBJECTIVES: The increasing evidence of the benefits of neonatal screening for cystic fibrosis (CF) indicates that this procedure could soon be implemented throughout France. The screening strategy currently used involves the detection of infants with elevated levels of immunoreactive trypsinogen (IRT) (approximately 1% of the population), followed by the detection of CFTR gene mutations. However, genetic analysis has certain drawbacks, the most important of which being the management of heterozygotes, and in France the requirement by law of previous informed consent. In cases of CF, pancreatic alterations are already present in utero. A previous study has demonstrated the value of pancreatitis-associated protein (PAP) as a screening test for CF, and has indicated that a feasible two-stage strategy could involve the following: 1) selection of infants with elevated PAP levels; 2) in this group of infants, subsequent detection of those with elevated IRT levels for direct CF diagnosis by the sweat test thereby avoiding the use of genetic analysis. The study aim was to evaluate this strategy in a large number of neonates. METHODS AND RESULTS: The aforementioned strategy was evaluated in a prospective study involving 47,213 infants in the Provence region of France. In infants with a PAP > 7.5 ng/mL (1.28%), 176 had an elevated IRT level > 700 ng/mL (0.37%). In this limited population sample (0.37% of the total), the sweat test diagnosed five cases of CF. A sixth case involving the monozygous twin of an infant with diagnosed CF remained undetected, probably because of a registration error. Genetic analysis confirmed the diagnosis, and also detected another case in an infant with two CFTR mutations but with a normal phenotype at 20 months of age. As the observed incidence was similar to that which had previously been reported, and as no further case was subsequently detected two years after the end of the study, this indicated that the sensitivity of this screening strategy was satisfactory. Its specificity makes the direct diagnosis of CF cases by the sweat test feasible, without further selection by genetic analysis. CONCLUSION: The PAP/IRT technique for CF detection seems to be suitable for mass screening, without the drawbacks of genetic testing.     

Combining immunoreactive trypsinogen and pancreatitis-associated protein assays, a method of newborn screening for cystic fibrosis that avoids DNA analysis

OBJECTIVES: To evaluate the performance of a strategy in which, after immunoreactive trypsinogen (IRT) determination, genetic analysis is replaced by a biological test, the pancreatitis-associated protein (PAP) enzyme-linked immunosorbent assay (ELISA). STUDY DESIGN: The French newborn screening program includes cystic fibrosis (CF) screening by the IRT/CFTR mutation strategy. PAP was assayed on screening cards, in parallel with IRT, in all newborns from 5 French regions (n = 204,749). Analysis of PAP values in CF and non-CF newborns with elevated IRT allowed direct comparison between the current strategy and the proposed IRT/PAP strategy. RESULTS: A protocol in which newborns with IRT >50 ng/mL and PAP >1.8 ng/mL and those with IRT >100 ng/mL and PAP >1.0 ng/mL are directly recalled for sweat testing would have the same performance as the IRT/CFTR mutation strategy. CONCLUSIONS: The IRT/PAP strategy is an alternative for CF newborn screening, which avoids the drawbacks of genetic analysis and is cheaper and easier to implement than the current IRT/CFTR mutation strategy.     

Development of a screening system for cystic fibrosis: meconium or blood spot trypsin assay or both?

High blood trypsin levels during early days of life are found in newborns subsequently diagnosed to be affected by cystic fibrosis. The authors compared the validity of the traditional meconium test with the blood immunoreactive trypsin (IRT) assay, carried out in parallel on 113,302 neonates from three regions of North-eastern Italy. The meconium test showed a sensitivity of 57.7%. The sensitivity of the IRT test was higher (96.1%). It was possible to identify by IRT 10 out of 11 false negative CFs at the meconium test. A shortcoming of neonatal IRT, however, is its low specificity; 1.6% of the newborns had to be retested. A new screening policy was therefore proposed and carried out on 69,640 newborns: the Lactase test (LACT) on meconium was introduced as a complementary assay in IRT positive newborns. If LACT exceeded 2 U/g dry meconium, a confirmatory sweat test was immediately requested; if LACT test was negative and IRT exceeded 85 micrograms/l, IRT was repeated. Postneonatal retesting values above 25 micrograms/l required a sweat test. As a result, the estimated prevalence of CF was 1:4,352, the sensitivity was 93.3%; the specificity turned out to be 99.6%, considering all false positive newborns investigated with retesting and/or direct sweat test.     

The CF-CIRC study: a French collaborative study to assess the accuracy of Cystic Fibrosis diagnosis in neonatal screening

Background  

Cystic fibrosis (CF) is caused by mutations in the gene encoding for the CF transmembrane conductance regulator (CFTR) protein, which acts as a chloride channel after activation by cyclic AMP (cAMP). Newborn screening programs for CF usually consist of an immunoreactive trypsinogen (IRT) assay, followed when IRT is elevated by testing for a panel of CF-causing mutations. Some children, however, may have persistent hypertrypsinogenemia, only one or no identified CFTR gene mutation, and sweat chloride concentrations close to normal values. In vivo demonstration of abnormal CFTR protein function would be an important diagnostic aid in this situation. Measurements of transepithelial nasal potential differences (NPD) in adults accurately characterize CFTR-related ion transport. The aim of the present study is to establish reference values for NPD measurements for healthy children and those with CF aged 3 months to 3 years, the age range of most difficult-to-diagnose patients with suspected CF. The ultimate goal of our study is to validate NPD testing as a diagnostic tool for children with borderline results in neonatal screening.     

Development of a Screening System for Cystic Fibrosis: Meconium or Blood Spot Trypsin Assay or Both?

ABSTRACT. High blood trypsin levels during early days of life are found in newborns subsequently diagnosed to be affected by cystic fibrosis. The authors compared the validity of the traditional meconium test with the blood immunoreactive trypsin (IRT) assay, carried out in parallel on 113302 neonates from three regions of North-eastern Italy. The meconium test showed a sensitivity of 57.7%. The sensitivity of the IRT test was higher (96.1 %). It was possible to identify by IRT 10 out of 11 false negative CFs at the meconium test. A shortcoming of neonatal IRT, however, is its low specificity; 1.6% of the newborns had to be retested. A new screening policy was therefore proposed and carried out on 69 640 newborns: the Lactase test (LACT) on meconium was introduced as a complementary assay in IRT positive newborns. If LACT exceeded 2 U/g dry meconium, a confirmatory sweat test was immediately requested; if LACT test was negative and IRT exceeded 85 μg/I, IRT was repeated. Postneonatal retesting values above 25 μg/l required a sweat test. As a result, the estimated prevalence of CF was 1:4352, the sensitivity was 93.3%; the specificity turned out to be 99.6%, considering all false positive newborns investigated with retesting and/or direct sweat test.     

Parental attitudes to the identification of their infants as carriers of cystic fibrosis by newborn screening

AIM: To investigate parental attitudes to cystic fibrosis (CF) carrier detection of their infant by newborn screening (NBS). METHODS: Data were collected from a postal questionnaire sent to parents of infants identified as CF carriers by NBS in 1996-1997 (inclusive) and 2001 in Victoria, Australia (n = 66). RESULTS: Almost all parents remembered their child being identified as a CF carrier (97%: 1996/1997; 100%: 2001); yet the majority were unaware at the time that NBS could detect carriers (70%: 1996/1997; 49%: 2001). More parents in the later cohort reported having carrier testing compared with the earlier cohort (85% and 53% respectively) but recall was more uncertain in the earlier cohort when validated against health records. Cascade testing was not utilised frequently by other family members in either cohort. Residual risk of being a carrier if testing was negative was not well understood by parents. Some parents (28%: 1996/1997; 18%: 2001) had residual anxiety about the current health of their carrier child and their future reproductive decision making. Most parents were satisfied with the information provided to them at the time of the sweat test. Few differences were seen between the cohorts. CONCLUSION: Although the NBS process for CF in Victoria is working efficiently for the majority of families whose infant is identified as a carrier there are areas that can be improved. We recommend that greater attention should be given to informing parents that a consequence of NBS is CF carrier detection and strategies to improve utilisation of cascade testing should be developed.     

Prospective and parallel assessments of cystic fibrosis newborn screening protocols in the Czech Republic: IRT/DNA/IRT versus IRT/PAP and IRT/PAP/DNA

Cystic fibrosis (CF) is a life-threatening disease for which early diagnosis following newborn screening (NBS) improves the prognosis. We performed a prospective assessment of the immunoreactive trypsinogen (IRT)/DNA/IRT protocol currently in use nationwide, versus the IRT/pancreatitis-associated protein (PAP) and IRT/PAP/DNA CF NBS protocols. Dried blood spots (DBS) from 106,522 Czech newborns were examined for IRT concentrations. In the IRT/DNA/IRT protocol, DNA-testing was performed for IRT?≥?65 ng/mL. Newborns with IRT?≥?200 ng/mL and no detected cystic fibrosis transmembrane conductance regulator gene (CFTR) mutations were recalled for a repeat IRT. In the same group of newborns, for both parallel protocols, PAP was measured in DBS with IRT?≥?50 ng/mL. In PAP-positive newborns (i.e., ≥1.8 if IRT 50-99.9 or ≥1.0 if IRT?≥?100, all in ng/mL), DNA-testing followed as part of the IRT/PAP/DNA protocol. Newborns with at least one CFTR mutation in the IRT/DNA/IRT and IRT/PAP/DNA protocols; a positive PAP in IRT/PAP; or a high repeat IRT in IRT/DNA/IRT were referred for sweat testing. CONCLUSION: the combined results of the utilized protocols led to the detection of 21 CF patients, 19 of which were identified using the IRT/DNA/IRT protocol, 16 using IRT/PAP, and 15 using IRT/PAP/DNA. Decreased cut-offs for PAP within the IRT/PAP protocol would lead to higher sensitivity but would increase false positives. Within the IRT/PAP/DNA protocol, decreased PAP cut-offs would result in high sensitivity, an acceptable number of false positives, and would reduce the number of DNA analyses. Thus, we concluded that the IRT/PAP/DNA protocol would represent the most suitable protocol in our conditions.     

3 Jahre Mukoviszidose-Screening für Neugeborene in Mecklenburg-Vorpommern

Background

Since October 2012, newborn screening (NBS) in the state of Mecklenburg-Western Pomerania (MV) was extended by the screening for cystic fibrosis (CF). The gained experiences are being shared for the nationwide implementation, which was decided by the G?BA in August 2015.

Method

The CF-NBS is being performed in two steps, but only when the parents signed a written consent for this additional screening offer. In the first step, the immunoreactive trypsinogen (IRT) is determined. When elevated, the pancreatitis associated protein (PAP) is analyzed in a second step. Newborns, considered as CF-NBS positive were called in for a sweat test in one of four existing certified CF ambulances in MV.

Results

Within these 36 months (October 2012–September 2015) a total of 40.294 newborns were born in MV and for 37.103 newborns, parents gave their written consent for the additional CF-NBS. This represents an average participation rate of > 95?%. For 740 of these 37.103 newborns (1,99?%), elevated IRT concentrations were detected and further 181 newborns had increased PAP values. Seven newborns with CF were diagnosed.

Discussion

The statewide implementation of the CF-NGS was successful. The challenges for the nationwide implementation are nevertheless divers. The G?BA has decided to implement an additional molecular genetic screening protocol step and the CF-NGS faces therefor a tight schedule. Furthermore, reimbursement concerning the sweat tests, the additional laboratory analytic and the tracking are not solved yet.

     

Newborn screening programmes for cystic fibrosis

We review the current situation with respect to newborn cystic fibrosis screening (NCFS) across the world. The challenges of establishing an NCFS programme are reflected in the diversity of those programmes identified. All employ an initial immunoreactive trypsinogen (IRT) measurement during the first week of life. If this is positive, a second IRT analysis at 4 weeks of age improves the specificity of the test; most programmes have, however, moved to DNA analysis at this point, which improves their sensitivity. Incorporating DNA analysis results in the identification of carriers, which may have implications for families but is generally considered positive. Some programmes have incorporated a second IRT test as well as DNA analysis, either to increase the sensitivity of the test in an area with a low DeltaF508 frequency or to reduce the number of infants requiring a sweat test. Whichever algorithm is selected, the implementation of a successful programme relies on clear pathways and good information for parents.     

Markedly elevated neonatal immunoreactive trypsinogen levels in the absence of cystic fibrosis gene mutations is not an indication for further testing.

AIMS: To investigate the immunoreactive trypsinogen (IRT) values above the usual 99th centile laboratory cut-off and determine the value of offering further testing to those infants with a markedly elevated IRT but no cystic fibrosis transmembrane regulator (CFTR) gene mutation identified by the screening programme. METHODS: All babies born in Victoria, Australia, between 1991 and 2003, were screened by IRT followed by CF gene mutation analysis. RESULTS: Of the 806,520 babies born, 9268 with the highest IRT levels had CFTR mutation analysis. There were 123 DeltaF508 homozygotes and 703 heterozygotes (86 with CF, 617 carriers). A total of 8442 babies had no CFTR gene mutation, of whom 18 (0.21%) had CF. The total number of CF babies with IRT greater than the laboratory cut-off was 227 (2.4%). The IRT results of the CF patients were distributed normally, with the majority above the laboratory cut-off of newborn IRT results. There was no evidence of an excess of babies with CF in the very highest levels of IRT above the 99th centile. CONCLUSIONS: Only a small proportion of babies with a neonatal IRT >99th centile have CF. Additional CF testing for infants with an elevated IRT but no CFTR gene mutation has an extremely low yield, no matter how high the IRT result.     

Markedly elevated neonatal immunoreactive trypsinogen levels in the absence of cystic fibrosis gene mutations is not an indication for further testing

Aims

To investigate the immunoreactive trypsinogen (IRT) values above the usual 99th centile laboratory cut‐off and determine the value of offering further testing to those infants with a markedly elevated IRT but no cystic fibrosis transmembrane regulator (CFTR) gene mutation identified by the screening programme.

Methods

All babies born in Victoria, Australia, between 1991 and 2003, were screened by IRT followed by CF gene mutation analysis.

Results

Of the 806 520 babies born, 9268 with the highest IRT levels had CFTR mutation analysis. There were 123 ΔF508 homozygotes and 703 heterozygotes (86 with CF, 617 carriers). A total of 8442 babies had no CFTR gene mutation, of whom 18 (0.21%) had CF. The total number of CF babies with IRT greater than the laboratory cut‐off was 227 (2.4%). The IRT results of the CF patients were distributed normally, with the majority above the laboratory cut‐off of newborn IRT results. There was no evidence of an excess of babies with CF in the very highest levels of IRT above the 99th centile.

Conclusions

Only a small proportion of babies with a neonatal IRT >99th centile have CF. Additional CF testing for infants with an elevated IRT but no CFTR gene mutation has an extremely low yield, no matter how high the IRT result.     

Delayed diagnosis of cystic fibrosis in children with a rare genotype (ΔF508/R117H)

Objectives: In neonatal screening for cystic fibrosis (CF), infants recognised as ΔF508 heterozygotes require a sweat test to confirm the diagnosis. However, compound heterozygotes with ΔF508 and the R117H mutation are known to have non-diagnostic sweat chlorides (<60 mmol/L) at an early age. As genotyping for rare mutations is not readily available in Australia, there is a need to determine whether quantitative pancreatic stimulation tests could facilitate the diagnosis of CF in three infants with the ΔF508/R117H mutation.
Methodology Formal sweat testing, genotyping and pancreatic stimulation tests were performed in three subjects heterozygous for ΔF508 who initially had non-diagnostic sweat chloride results (40-60 mmol/L) but presented later with persisting chest symptoms and/or signs consistent with CF.
Results All three patients were shown to have the ΔF508/R117H genotype with initial sweat chloride results ranging from 40 to 58 mmol/L. Pancreatic stimulation tests demonstrated reduced enzyme secretion in two and decreased fluid, bicarbonate and chloride secretion in all three patients.
Conclusions In infants recognized as ΔF508 heterozygotes by the newborn screening programme, the presence of an equivocal sweat chloride does not exclude the diagnosis of CF. If such patients with an initially equivocal sweat chloride subsequently develop symptoms suggestive of CF and have a persisting non-diagnostic sweat chloride then the diagnosis of CF can be confirmed by more extensive genotyping if available or by pancreatic stimulation testing.