title: Clinically helpful rickettsial disease diagnostic IgG titers in relation to duration of illness in an endemic setting in Sri Lanka.
abstract: BACKGROUND
  Although an initial IFA-IgG titer greater or equal to 1/64 or 1/128 is considered positive in presumptive diagnosis, in clinical practice in an endemic setting for rickettsioses in Sri Lanka, some patients with IFA-IgG titer of 1/128 for either spotted fever group (SFG) or scrub typhus (ST) did not respond to treatment.
  
  
  RESULTS
  To determine a clinically helpful diagnostic algorithm, IFA-IgG results of serologically confirmed treatment responders were analyzed in relation to duration of illness at sampling. Of 146 suspected SFG, 3 responders of 25 patients had titers ≤1/128 with < 7 days of illness while all 9 with titers ≥1/256 responded (false negative with 1/256 cutoff was 12%, false positive was 0%). For illness > 7 days, the false negative and positive rates were 4.3% (3/59) and 11.3% (6/53). Of 115 suspected ST, false negative and positive rates with ≥1/256 cutoff at <7 days of illness were 14.2% (2/14) and 0% (0/8) respectively while > 7 days, false negative and positive rates were 2% (1/51) and 0% (0/42).
  
  
  CONCLUSIONS
  For clinical decision making, duration of illness at sampling is important in interpreting serology results in an endemic setting. If sample is obtained ≤7 day of illness, an IgG titer of ≤1/128 requires a follow up sample in the diagnosis and > 7 days of illness, a single ≥1/256 titer is diagnostic for all ST and 90% of SFG.
authors.affiliation: Department of Medicine, Faculty of Medicine, University of Kelaniya, Colombo, Sri Lanka. ranjanp64@gmail.com
authors.forename: Ranjan
authors.initials: R
authors.lastname: Premaratna
authors.forename: Sanjaya
authors.initials: S
authors.lastname: Weerasinghe
authors.forename: Amanda
authors.initials: A
authors.lastname: Ranaweera
authors.forename: T G A Nilmini
authors.initials: TG
authors.lastname: Chandrasena
authors.forename: Narasinghe W
authors.initials: NW
authors.lastname: Bandara
authors.forename: Gregory A
authors.initials: GA
authors.lastname: Dasch
authors.forename: H Janaka
authors.initials: HJ
authors.lastname: de Silva
mesh_terms: D000465:Algorithms; D000907:Antibodies, Bacterial / Q000097:blood*; D015415:Biomarkers / Q000097:blood; D001907:Boutonneuse Fever / Q000175:diagnosis / Q000276:immunology / Q000382:microbiology; D003661:Decision Support Techniques; D019353:Endemic Diseases; D005188:False Negative Reactions; D005189:False Positive Reactions; D005455:Fluorescent Antibody Technique; D006801:Humans; D007074:Immunoglobulin G / Q000097:blood; D012285:Orientia tsutsugamushi / Q000276:immunology; D011237:Predictive Value of Tests; D012189:Retrospective Studies; D012281:Rickettsia / Q000276:immunology*; D012282:Rickettsia Infections / Q000097:blood / Q000175:diagnosis* / Q000453:epidemiology / Q000276:immunology / Q000382:microbiology; D020114:Rickettsia conorii / Q000276:immunology; D012612:Scrub Typhus / Q000175:diagnosis / Q000276:immunology / Q000382:microbiology; D013188:Sri Lanka / Q000453:epidemiology; D013997:Time Factors
chemical_list: D000907:Antibodies, Bacterial; D015415:Biomarkers; D007074:Immunoglobulin G