title: Confirmatory assays are essential when using molecular testing for Neisseria gonorrhoeae in low-prevalence settings: insights from the third National Survey of Sexual Attitudes and Lifestyles (Natsal-3).
abstract: OBJECTIVE
  To investigate the occurrence of unconfirmed positive gonorrhoea results when using molecular testing within a large population-based survey.
  
  
  METHODS
  Between 2010 and 2012, we did a probability sample survey of 15,162 men and women aged 16-74 years in Britain. Urine from participants aged 16-44 years reporting ≥1 lifetime sexual partner was tested for Neisseria gonorrhoeae and Chlamydia trachomatis using the Aptima Combo 2 (AC2) assay, with positive or equivocal results confirmed with molecular assays using different nucleic acid targets.
  
  
  RESULTS
  A total of 4550 participants aged 16-44 years had urine test results (1885 men; 2665 women). For gonorrhoea, 18 samples initially tested positive and eight were equivocal. Only five out of 26 confirmed, giving a positive predictive value (PPV) for the initial testing of 19% (95% CI 4% to 34%). Most (86% (18/21)) participants with unconfirmed positive results for gonorrhoea reported zero or one sexual partner without condoms in the past year and none had chlamydia co-infection, whereas all five with confirmed gonorrhoea reported at least two recent sexual partners without condoms, and four had chlamydia co-infection. The weighted prevalence for gonorrhoea positivity fell from 0.4% (0.3% to 0.7%) after initial screening to <0.1% (0.0% to 0.1%) after confirmatory testing. By comparison, 103 samples tested positive or equivocal for chlamydia and 98 were confirmed (PPV=95% (91% to 99%)).
  
  
  CONCLUSIONS
  We highlight the low PPV for gonorrhoea of an unconfirmed reactive test when deploying molecular testing in a low-prevalence population. Failure to undertake confirmatory testing in low-prevalence settings may lead to inappropriate diagnoses, unnecessary treatment and overestimation of population prevalence.
authors.affiliation: Research Department of Infection & Population Health, University College London, Mortimer Market Centre, London, UK.
authors.forename: Nigel
authors.initials: N
authors.lastname: Field
authors.affiliation: Research Department of Infection & Population Health, University College London, Mortimer Market Centre, London, UK NatCen Social Research, London, UK.
authors.forename: Soazig
authors.initials: S
authors.lastname: Clifton
authors.affiliation: Sexually Transmitted Bacteria Reference Unit, Public Health England, London, UK.
authors.forename: Sarah
authors.initials: S
authors.lastname: Alexander
authors.affiliation: Sexually Transmitted Bacteria Reference Unit, Public Health England, London, UK.
authors.forename: Catherine A
authors.initials: CA
authors.lastname: Ison
authors.affiliation: Public Health England, National Centre for Infectious Disease Surveillance and Control, London, UK.
authors.forename: Gwenda
authors.initials: G
authors.lastname: Hughes
authors.affiliation: Virus Reference Department, Public Health England, London, UK.
authors.forename: Simon
authors.initials: S
authors.lastname: Beddows
authors.affiliation: Research Department of Infection & Population Health, University College London, Mortimer Market Centre, London, UK.
authors.forename: Clare
authors.initials: C
authors.lastname: Tanton
authors.affiliation: Public Health England, National Centre for Infectious Disease Surveillance and Control, London, UK.
authors.forename: Kate
authors.initials: K
authors.lastname: Soldan
authors.affiliation: Virus Reference Department, Public Health England, London, UK.
authors.forename: Filomeno
authors.initials: F
authors.lastname: Coelho da Silva
authors.affiliation: Research Department of Infection & Population Health, University College London, Mortimer Market Centre, London, UK.
authors.forename: Catherine H
authors.initials: CH
authors.lastname: Mercer
authors.affiliation: Department of Social and Environmental Research, London School of Hygiene & Tropical Medicine, London, UK.
authors.forename: Kaye
authors.initials: K
authors.lastname: Wellings
authors.affiliation: Research Department of Infection & Population Health, University College London, Mortimer Market Centre, London, UK.
authors.forename: Anne M
authors.initials: AM
authors.lastname: Johnson
authors.affiliation: Research Department of Infection & Population Health, University College London, Mortimer Market Centre, London, UK.
authors.forename: Pam
authors.initials: P
authors.lastname: Sonnenberg
mesh_terms: D000293:Adolescent; D000328:Adult; D002690:Chlamydia Infections / Q000453:epidemiology*; D002692:Chlamydia trachomatis / Q000302:isolation & purification*; D005260:Female; D006069:Gonorrhea / Q000453:epidemiology*; D006306:Health Surveys; D006801:Humans; D008019:Life Style; D008297:Male; D008875:Middle Aged; D009344:Neisseria gonorrhoeae / Q000302:isolation & purification*; D021141:Nucleic Acid Amplification Techniques; D011159:Population Surveillance; D015995:Prevalence; D012307:Risk Factors; D012725:Sexual Behavior; D006113:United Kingdom / Q000453:epidemiology
keywords: EPIDEMIOLOGY (CLINICAL); NEISSERIA GONORRHOEA; PUBLIC HEALTH; TESTING