Skip to main content

Table 1 Model parameters

From: Impact and cost-effectiveness of chlamydia testing in Scotland: a mathematical modelling study

Parameter Symbol Baseline value Range Source Notes
Chlamydia natural history
Transmission probability per partnership β 0.346 per partner -- Calibrated by model fitting Estimated by fitting model predictions to chlamydia prevalence and overall testing coverage. Modelled by fixing duration of infection and allowing transmission probability to vary – method also used by Althaus [24] and Clarke [25]
Rate of recovery from infection per year δ 1 per yr -- [9, 10] No estimates in men; likely shorter than in women. Estimates in women from recent modelling studies: 14 months [9] and 16 months [10]. Assumed shorter duration overall to include men. Model transmission modified to fit to desired prevalence, and transmission probability and duration correlated
Risk of PID in those with incident chlamydia PID_risk 0.16 0.06-0.25 [17] Range derived from literature estimates
Risk of TFI in those with incident chlamydia TFI_risk 0.02 0.01-0.04 [26] Range derived from literature estimates
Demography
Female population in Scotland aged 15–24 years N TARGET_F 335,518 -- [5] Population estimate as at 30 June 2010
Male population in Scotland aged 15–24 years N TARGET_M 349,417 -- [5] Population estimate as at 30 June 2010
Rate of entry into the model per year φ 1/10 per yr --   
Rate of ageing from model per year α 1/10 per yr --   
Sexual behaviour
Proportion recruited into activity group i r i r[1] = 0.702; r[2] = 0.230; r[3] = 0.068 -- [23]  
Partner contact rate per year in those in activity group i c i c[1] = 0.674; c[2] = 2.538; c[3] = 9.452 -- [23]  
Mixing between sexual activity classes ϵ 0.2 -- [27] Based on previous estimates where 0 represents proportionate mixing and 1 fully assortative mixing
Testing and treatment
Baseline prevalence among females and males aged 15–24 years PREV 4.4% -- [8] Based on prevalence among 16–24 year olds in Scotland (Scottish-specific prevalence data kindly provided by Natsal-3 researchers)
Overall testing coverage TEST 16.8% 8.4%, 16.8%, 25.2%, 33.6%, 42.0 Stepwise values (0.5, 1, 1.5, 2, 2.5 increases relative to baseline) across an assumed realistic range Note: the overall testing coverage includes all types of test (additional testing, treatment seeking and partner notification) and is the coverage at baseline partner notification efficacy (0.4). Changes in partner notification result in small changes in overall coverage but which are not shown on the figures for simplicity
Additional testing coverage COV 11.9% 2.3%, 11.9%, 21.5%, 31.0%, 40.5% Calibrated by model fitting Estimated from fitting model predictions to chlamydia prevalence and overall testing coverage
Percentage of additionally tested individuals, or individuals seeking treatment (females or males) identified as positive who are successfully treated D TREATED 91% -- NCSP 2011-2012 [28] NCSP target is 95%. It is assumed that treatment is only given after a positive test result, and that there can therefore be loss to follow-up between testing and treatment. This does not include treatment failure, which is not incorporated in the model
Treatment seeking behaviour
Proportion of all those infected who seek treatment SEEKTREAT INF 0.2 -- [24] Selected for convenience to differentiate treatment seeking behaviour which is not dependent on policy i.e., based on symptoms or contact with infected partner, and testing of asymptomatic individuals that could be modified depending on testing strategy adopted. Althaus et al. found that the value of the proportion symptomatic and seeking treatment a short time after infection did not have a substantial effect on transmission dynamics
Proportion of all those seeking treatment who are infected with chlamydia POS SEEKTREAT 0.2 -- Assumed realistic value Chosen to be slightly lower than assumed prevalence among partners
Partner notification
Number of partners successfully notified and tested/treated per treated index (from either additional testing or treatment seeking testing) (=partner notification efficacy) PNe 0.4 0.0-2.0 in 0.25 increments [29] Range within the number of partners reported by index cases (e.g., NCSP range 0.1-1 partner notified per index)
Percentage positive among partners tested POS PN 30% -- [30]  
Costs
Cost of a test, including treatment for those positive (average cost) C TEST £45 Percentage change: -50% to +100% [31] Does not vary with population prevalence. NAO says it should be possible to do a test for £33 [32]
Cost of partner notification per partner, including testing and treatment for those positive (average cost) C PN £114 Percentage change: -50% to +100% [31] Does not vary with population prevalence
Cost of treating PID C PID £163 -- [33]  
Cost of treating TFI C TFI £2,115 -- [34] Cost of one round of IVF on the NHS (conservatively costed in order to account for those infertile women who do not undergo IVF)
Health state utility      
PID -- 0.9 Percentage change: -50% to +100% [22] Applies for 3 months
TFI -- 0.76 Percentage change: -50% to +100% [22] Applies for 1 year