Table 1

Model input parameters: epidemiological and clinical

 VariableValueNumberCommentsReference/calculation
MenWomenMenWomen
ANumber of STI screens47.3%52.7%595 802662 904Chlamydia and gonorrhoea testGUMCAD 2011, table 51
BChlamydial infection8.6%7.4%51 35249 295Total diagnosesGUMCAD 2011, table 51
CGonorrhoeal infection2.5%0.9%14 9925972Total diagnosesGUMCAD 2011, table 51
DProportion symptomatic35%48%208 531318 194Symptomatic pathwayMSTIC study C Mercer*, personal communication*16
EProportion asymptomatic65%52%Asymptomatic pathway
Chlamydia
FRelative risk (RR) of chlamydia in symptomatic1.70.6Derived from MSTIC studyC Mercer*, personal communication16
GProportion of asymptomatic infected6.9%9.0%26 85531 011Calculated from RR (row F)Symptomatic positivity = RRxAsymptomatic positivity
HProportion symptomatic infected11.7%5.7%24 49718 284Calculated from RR (row F)
IProportion infected symptomatic presumptively treated70%24%17 1484388Estimate (correct presumptive)Assumption
JProportion uninfected symptomatic presumptively treated33%8%60 73121 512Estimate (overtreatment)Assumption
KProportion of symptomatic presumptively treated37%8%77 87925 900Calculated (Col 4 or 5)=(I+J)/E
LProportion of presumptively treated infected22%17%Calculated (Col 4 or 5)=I/(I+J)
Gonorrhoea
MRR of gonorrhoea in symptomatic4.50.8Derived from MSTIC studyC Mercer, personal communication16
NProportion asymptomatic infected1.1%1.0%43683406Calculated using RR (row M)
OProportion symptomatic infected5.1%0.8%10 6242566Calculated using RR (row M)
PProportion infected symptomatic presumptively treated90%50%95621283Correct presumptiveAssumption
QProportion uninfected symptomatic presumptively treated2%3%39589469OvertreatmentBroadly consistent with GUMCAD1†
RProportion of symptomatic presumptively treated6%3%13 52010 752Calculated (Col 4 or 5)=(P+Q)/E
SProportion of presumptively treated infected71%12%Calculated (Col 4 or 5)=P/(P+Q)
Other parameters
TProportion of GUM attendees who present as contacts of infected who are presumptively treated100%100%BASHH guidelines2 3
UTransmission probability per sex act chlamydia (no condom)5%5%During unprotected sex acts in 2 weeks following GUM visitConservative estimate17
VTransmission probability per sex act gonorrhoea (no condom)10%10%Conservative estimate18
WNumber of unprotected sex acts per week after GUM visit22Conservative estimate
XProgression to PID from chlamydia (per day)00.00035Estimated from Bayesian evidence synthesis19‡
YProgression to PID from gonorrhoea (per day)00.00035Assumed same as chlamydia
ZTreatment effectiveness95%95%EstimateGuidelines require >95% efficacy2 3
AAProbability that partner of index is chlamydia positive0.40.4Conservative assumptionAssumption
ABProbability that partner of index is gonorrhoea positive0.40.4Conservative assumptionAssumption

  • *We use RR to adjust the fraction of infections occurring in symptomatic or asymptomatic pathways but retained the overall prevalence as observed in GUMCAD. RR in symptomatic patients was calculated based on proportion infected in those reporting symptoms at attendance in MSTIC (533 men, 731 women)15 (unpublished data kindly provided by Cath Mercer, UCL). The RR was then applied to the GUMCAD data to distribute infections between those symptomatically and asymptomatically infected.

  • †The proportion of those uninfected who get treated presumptively was calculated such that the total amount of presumptive treatment is broadly consistent with reported epidemiological treatment in GUMCAD. Also see figure 1 for illustration of how these parameters play out in the influence diagram.

  • ‡Price et al19 recently synthesised evidence to calculate the overall progression rate from untreated chlamydia to PID as 0.16 (0.06 to 0.25 CIs). Assuming the mean duration of untreated chlamydia is 493 days and a constant risk of progression, this equates to a risk of 0.00035 per day, by rearranging the formula: y=1−(1−x)493 where y is the total incidence (y=0.16) to calculate x, the daily probability of progression to PID.

  • GUMCAD, Genitourinary Medicine Clinic Activity Dataset; PID, pelvic inflammatory disease; STI, sexually transmitted infection.