title: Screening for chromosomal anomalies in the first trimester: does repeat maternal serum screening improve detection rates?
abstract: OBJECTIVE
  To assess the within person biological variability of first trimester maternal serum biochemical markers of trisomy 21 across the 10-14 week gestational period. To evaluate whether repeat sampling and testing of free beta-hCG and PAPP-A during this period would result in an improved detection rate.
  
  
  METHODS
  Women presenting at the first trimester OSCAR clinic have blood collected prior to ultrasound dating and nuchal translucency measurement. All samples are analysed for free beta-hCG and PAPP-A before an accurate estimate of gestation is available. In 10% of cases the gestation is prior to the minimum time for NT measurement (11 weeks) and these women are rebooked for a repeat visit to the clinic at the appropriate time. Our fetal database was interrogated to obtain cases in which two maternal blood samples had been collected and analysed in the 10-14 week period. Using data from the marker correlations and statistical modelling, the impact of repeat testing on detection rate for trisomy 21 at a fixed 5% false positive rate, was assessed.
  
  
  RESULTS
  261 pairs of data were available for analysis collected over a 3 year period. The correlation between free beta-hCG in sample 1 and sample 2 was 0.890 and that for PAPP-A was 0.827. The average within person biological variation for free beta-hCG was 21% and 32% for PAPP-A. The increase in detection rate when using both sets of marker data was 3.5% when using serum biochemistry and maternal age, and 1.3% when using nuchal translucency, serum biochemistry and maternal age.
  
  
  CONCLUSIONS
  Repeat sampling and testing of maternal serum biochemical markers is unlikely to substantially improve first trimester screening performance.
authors.affiliation: Clinical Biochemistry Department, Harold Wood Hospital, Gubbins Lane, Romford, Essex RM3 0BE, UK. KevinSpencer1@aol.com
authors.forename: Kevin
authors.initials: K
authors.lastname: Spencer
authors.forename: Howard S
authors.initials: HS
authors.lastname: Cuckle
mesh_terms: D018997:Chorionic Gonadotropin, beta Subunit, Human / Q000097:blood; D002869:Chromosome Aberrations; D005189:False Positive Reactions; D005260:Female; D005865:Gestational Age; D006801:Humans; D008423:Maternal Age; D009333:Neck / Q000000981:diagnostic imaging / Q000196:embryology; D011247:Pregnancy; D011266:Pregnancy-Associated Plasma Protein-A / Q000032:analysis; D011296:Prenatal Diagnosis / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014463:Ultrasonography
chemical_list: D018997:Chorionic Gonadotropin, beta Subunit, Human; D011266:Pregnancy-Associated Plasma Protein-A