title: Colorectal cancer screening: why immunochemical faecal occult blood test performs as well with either one or two samples.
abstract: BACKGROUND
  Immunochemical faecal occult blood tests perform as well with either one or two samples, and better than guaiac tests with 6 samples.
  
  
  OBJECTIVE
  Clarifying relationship between tests' performance, bleeding pattern and observation level.
  
  
  METHODS
  The data of 32,225 average-risk subjects who performed both Hemoccult II (guaiac) and Magstream (immunochemical) tests were re-analysed by varying the cutoff and number of samples of Magstream.
  
  
  RESULTS
  The identical performances obtained using one or two samples of Magstream (lower cutoff for one sample) at the population level were explained by opposite patterns of bleeding according to the presence of advanced neoplasias. They translated into discrepancy at the individual level: for example a 60% increase in sensitivity and 20% in specificity observed with one (39ng Hb/ml cutoff) or two samples (63ng Hb/ml cutoff) Magstream compared with Hemoccult II meant that 28.5% of the subjects testing positive with one sample (18.0% in subjects with advanced neoplasias) would have been considered negative by using two samples of Magstream at a higher cutoff (and reciprocal).
  
  
  CONCLUSIONS
  The identical performance of immunochemical tests using one or two samples (different cutoff), explained by opposite pattern of bleeding according to advanced neoplasias is true only at the population level, the appropriate level for mass screening.
authors.affiliation: INSERM U1086 Cancers & Preventions, CHU de Caen, Caen, France. guittet-l@chu-caen.fr
authors.forename: Lydia
authors.initials: L
authors.lastname: Guittet
authors.forename: Véronique
authors.initials: V
authors.lastname: Bouvier
authors.forename: Elodie
authors.initials: E
authors.lastname: Guillaume
authors.forename: Romuald
authors.initials: R
authors.lastname: Levillain
authors.forename: Angela
authors.initials: A
authors.lastname: Ruiz
authors.forename: Olivier
authors.initials: O
authors.lastname: Lantieri
authors.forename: Guy
authors.initials: G
authors.lastname: Launoy
mesh_terms: D000236:Adenoma / Q000175:diagnosis*; D000368:Aged; D015179:Colorectal Neoplasms / Q000175:diagnosis*; D055088:Early Detection of Cancer / Q000379:methods*; D005260:Female; D006138:Guaiac; D006801:Humans; D007159:Immunologic Tests / Q000379:methods*; D008297:Male; D008403:Mass Screening / Q000379:methods; D008875:Middle Aged; D009780:Occult Blood; D012372:ROC Curve
chemical_list: D006138:Guaiac