title: Defining Seropositivity Thresholds for Use in Trachoma Elimination Studies.
abstract: Efforts are underway to eliminate trachoma as a public health problem by 2020. Programmatic guidelines are based on clinical signs that correlate poorly with Chlamydia trachomatis (Ct) infection in post-treatment and low-endemicity settings. Age-specific seroprevalence of anti Ct Pgp3 antibodies has been proposed as an alternative indicator of the need for intervention. To standardise the use of these tools, it is necessary to develop an analytical approach that performs reproducibly both within and between studies.
  
  
  
  Dried blood spots were collected in 2014 from children aged 1-9 years in Laos (n = 952) and Uganda (n = 2700) and from people aged 1-90 years in The Gambia (n = 1868). Anti-Pgp3 antibodies were detected by ELISA. A number of visual and statistical analytical approaches for defining serological status were compared.
  
  
  
  Seroprevalence was estimated at 11.3% (Laos), 13.4% (Uganda) and 29.3% (The Gambia) by visual inspection of the inflection point. The expectation-maximisation algorithm estimated seroprevalence at 10.4% (Laos), 24.3% (Uganda) and 29.3% (The Gambia). Finite mixture model estimates were 15.6% (Laos), 17.1% (Uganda) and 26.2% (The Gambia). Receiver operating characteristic (ROC) curve analysis using a threshold calibrated against external reference specimens estimated the seroprevalence at 6.7% (Laos), 6.8% (Uganda) and 20.9% (The Gambia) when the threshold was set to optimise Youden's J index. The ROC curve analysis was found to estimate seroprevalence at lower levels than estimates based on thresholds established using internal reference data. Thresholds defined using internal reference threshold methods did not vary substantially between population samples.
  
  
  
  Internally calibrated approaches to threshold specification are reproducible and consistent and thus have advantages over methods that require external calibrators. We propose that future serological analyses in trachoma use a finite mixture model or expectation-maximisation algorithm as a means of setting the threshold for ELISA data. This will facilitate standardisation and harmonisation between studies and eliminate the need to establish and maintain a global calibration standard.
authors.affiliation: Clinical Research Department, London School of Hygiene & Tropical Medicine, London, United Kingdom.
authors.forename: Stephanie J
authors.identifier: 0000-0003-2049-9040
authors.initials: SJ
authors.lastname: Migchelsen
authors.affiliation: Division of Parasitic Diseases and Malaria, Centers for Disease Control and Prevention, Atlanta, Georgia, United States of America.
authors.forename: Diana L
authors.initials: DL
authors.lastname: Martin
authors.affiliation: Center for Ophthalmology, Ministry of Health, Vientiane, Lao People's Democratic Republic.
authors.forename: Khamphoua
authors.initials: K
authors.lastname: Southisombath
authors.affiliation: Trachoma Control Programme, Ugandan Ministry of Health, Kampala, Uganda.
authors.forename: Patrick
authors.initials: P
authors.lastname: Turyaguma
authors.affiliation: NTD Support Centre, Decatur, Georgia, United States of America.
authors.forename: Anne
authors.initials: A
authors.lastname: Heggen
authors.affiliation: ENVISION PROGRAMME, RTI International, Kampala, Uganda.
authors.forename: Peter Paul
authors.initials: PP
authors.lastname: Rubangakene
authors.affiliation: Medical Research Council Unit The Gambia, Fajara, The Gambia.
authors.forename: Hassan
authors.initials: H
authors.lastname: Joof
authors.affiliation: Medical Research Council Unit The Gambia, Fajara, The Gambia.
authors.forename: Pateh
authors.initials: P
authors.lastname: Makalo
authors.affiliation: Division of Parasitic Diseases and Malaria, Centers for Disease Control and Prevention, Atlanta, Georgia, United States of America.
authors.forename: Gretchen
authors.initials: G
authors.lastname: Cooley
authors.affiliation: IHRC, Inc., Centers for Disease Control and Prevention, Atlanta, Georgia, United States of America.
authors.forename: Sarah
authors.initials: S
authors.lastname: Gwyn
authors.affiliation: Clinical Research Department, London School of Hygiene & Tropical Medicine, London, United Kingdom.|Department of Control of Neglected Tropical Diseases, World Health Organization, Geneva, Switzerland.
authors.forename: Anthony W
authors.initials: AW
authors.lastname: Solomon
authors.affiliation: Clinical Research Department, London School of Hygiene & Tropical Medicine, London, United Kingdom.
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Holland
authors.affiliation: Kilimanjaro Centre for Community Ophthalmology, University Cape Town, Cape Town, South Africa.
authors.forename: Paul
authors.initials: P
authors.lastname: Courtright
authors.affiliation: International Trachoma Initiative, The Task Force for Global Health, Decatur, Georgia, United States of America.
authors.forename: Rebecca
authors.initials: R
authors.lastname: Willis
authors.affiliation: MRC Tropical Epidemiology Group, Infectious Disease Epidemiology Department, London School of Hygiene & Tropical Medicine, London, United Kingdom.
authors.forename: Neal D E
authors.initials: ND
authors.lastname: Alexander
authors.affiliation: Clinical Research Department, London School of Hygiene & Tropical Medicine, London, United Kingdom.
authors.forename: David C W
authors.initials: DC
authors.lastname: Mabey
authors.affiliation: Clinical Research Department, London School of Hygiene & Tropical Medicine, London, United Kingdom.
authors.forename: Chrissy H
authors.initials: CH
authors.lastname: Roberts
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000907:Antibodies, Bacterial / Q000097:blood*; D002648:Child; D002675:Child, Preschool; D002692:Chlamydia trachomatis / Q000235:genetics / Q000276:immunology* / Q000302:isolation & purification; D060740:Disease Eradication; D005260:Female; D006801:Humans; D007223:Infant; D008297:Male; D008875:Middle Aged; D014141:Trachoma / Q000097:blood / Q000453:epidemiology / Q000382:microbiology / Q000517:prevention & control*; D055815:Young Adult
chemical_list: D000907:Antibodies, Bacterial