title: Probabilities in norovirus outbreak diagnosis.
abstract: BACKGROUND
  Noroviruses are recognized as the most common cause of outbreaks of acute gastroenteritis. Yet, diagnostic testing for norovirus is based mostly on RNA detection by RT-PCR, which is not widely available. While antigen detection tests (ELISAs) are easier to perform, they are in general less sensitive.
  
  
  OBJECTIVE
  Our aim was to provide a scientific basis for declaring norovirus as the causative agent of an outbreak of acute gastroenteritis.
  
  
  METHODS
  Statistical analysis used binomial distribution to determine the minimal number of positive samples, and the probability of detecting the required number of positive samples, for different tests, required to assign norovirus as the causative agent of an outbreak of acute gastroenteritis.
  
  
  RESULTS
  For either a standard RT-PCR or a commercially available ELISA, finding only 1 sample positive out of 2, 3 or 4 samples is sufficient to assign norovirus as the causative agent of an outbreak of acute gastroenteritis. However, when ELISA is used, the probability of detecting this required minimum number of positive samples is low when small numbers of samples are tested (57% when 2 samples are tested; 72% when 3 samples are tested). In order to reach a 90% probability of detecting a norovirus outbreak (false negativity at outbreak level <10%), at least 3 samples should be tested using RT-PCR, and 6 samples when using an ELISA.
  
  
  CONCLUSIONS
  The sensitivity for NoV outbreak diagnosis will increase from 57% to 92%, or from 84% to 96%, for ELISA or RT-PCR respectively, when sample size increases from 2 to 6. Thus, using ELISA instead of RT-PCR for the detection of norovirus in stool samples will result in considerable numbers of false negative outbreaks unless a minimum of 6 samples are tested per outbreak.
authors.affiliation: Center for Infectious Diseases Control, National Institute for Public Health and the Environment (RIVM), Bilthoven, The Netherlands. Erwin.Duizer@RIVM.NL
authors.forename: Erwin
authors.initials: E
authors.lastname: Duizer
authors.forename: Annemarie
authors.initials: A
authors.lastname: Pielaat
authors.forename: Harry
authors.initials: H
authors.lastname: Vennema
authors.forename: Annelies
authors.initials: A
authors.lastname: Kroneman
authors.forename: Marion
authors.initials: M
authors.lastname: Koopmans
mesh_terms: D017250:Caliciviridae Infections / Q000175:diagnosis* / Q000453:epidemiology / Q000821:virology; D004196:Disease Outbreaks / Q000706:statistics & numerical data; D004797:Enzyme-Linked Immunosorbent Assay; D005243:Feces / Q000821:virology; D005759:Gastroenteritis / Q000175:diagnosis* / Q000453:epidemiology / Q000821:virology; D006801:Humans; D009426:Netherlands / Q000453:epidemiology; D029322:Norovirus / Q000302:isolation & purification*; D011336:Probability; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012680:Sensitivity and Specificity