Variable | Value | Number | Comments | Reference/calculation | |||
---|---|---|---|---|---|---|---|
Men | Women | Men | Women | ||||
A | Number of STI screens | 47.3% | 52.7% | 595 802 | 662 904 | Chlamydia and gonorrhoea test | GUMCAD 2011, table 51 |
B | Chlamydial infection | 8.6% | 7.4% | 51 352 | 49 295 | Total diagnoses | GUMCAD 2011, table 51 |
C | Gonorrhoeal infection | 2.5% | 0.9% | 14 992 | 5972 | Total diagnoses | GUMCAD 2011, table 51 |
D | Proportion symptomatic | 35% | 48% | 208 531 | 318 194 | Symptomatic pathway | MSTIC study C Mercer*, personal communication*16 |
E | Proportion asymptomatic | 65% | 52% | Asymptomatic pathway | |||
Chlamydia | |||||||
F | Relative risk (RR) of chlamydia in symptomatic | 1.7 | 0.6 | Derived from MSTIC study | C Mercer*, personal communication16 | ||
G | Proportion of asymptomatic infected | 6.9% | 9.0% | 26 855 | 31 011 | Calculated from RR (row F) | Symptomatic positivity = RRxAsymptomatic positivity |
H | Proportion symptomatic infected | 11.7% | 5.7% | 24 497 | 18 284 | Calculated from RR (row F) | |
I | Proportion infected symptomatic presumptively treated | 70% | 24% | 17 148 | 4388 | Estimate (correct presumptive) | Assumption |
J | Proportion uninfected symptomatic presumptively treated | 33% | 8% | 60 731 | 21 512 | Estimate (overtreatment) | Assumption |
K | Proportion of symptomatic presumptively treated | 37% | 8% | 77 879 | 25 900 | Calculated (Col 4 or 5) | =(I+J)/E |
L | Proportion of presumptively treated infected | 22% | 17% | Calculated (Col 4 or 5) | =I/(I+J) | ||
Gonorrhoea | |||||||
M | RR of gonorrhoea in symptomatic | 4.5 | 0.8 | Derived from MSTIC study | C Mercer, personal communication16 | ||
N | Proportion asymptomatic infected | 1.1% | 1.0% | 4368 | 3406 | Calculated using RR (row M) | |
O | Proportion symptomatic infected | 5.1% | 0.8% | 10 624 | 2566 | Calculated using RR (row M) | |
P | Proportion infected symptomatic presumptively treated | 90% | 50% | 9562 | 1283 | Correct presumptive | Assumption |
Q | Proportion uninfected symptomatic presumptively treated | 2% | 3% | 3958 | 9469 | Overtreatment | Broadly consistent with GUMCAD1† |
R | Proportion of symptomatic presumptively treated | 6% | 3% | 13 520 | 10 752 | Calculated (Col 4 or 5) | =(P+Q)/E |
S | Proportion of presumptively treated infected | 71% | 12% | Calculated (Col 4 or 5) | =P/(P+Q) | ||
Other parameters | |||||||
T | Proportion of GUM attendees who present as contacts of infected who are presumptively treated | 100% | 100% | BASHH guidelines2 3 | |||
U | Transmission probability per sex act chlamydia (no condom) | 5% | 5% | During unprotected sex acts in 2 weeks following GUM visit | Conservative estimate17 | ||
V | Transmission probability per sex act gonorrhoea (no condom) | 10% | 10% | Conservative estimate18 | |||
W | Number of unprotected sex acts per week after GUM visit | 2 | 2 | Conservative estimate | |||
X | Progression to PID from chlamydia (per day) | 0 | 0.00035 | Estimated from Bayesian evidence synthesis19‡ | |||
Y | Progression to PID from gonorrhoea (per day) | 0 | 0.00035 | Assumed same as chlamydia | |||
Z | Treatment effectiveness | 95% | 95% | Estimate | Guidelines require >95% efficacy2 3 | ||
AA | Probability that partner of index is chlamydia positive | 0.4 | 0.4 | Conservative assumption | Assumption | ||
AB | Probability that partner of index is gonorrhoea positive | 0.4 | 0.4 | Conservative assumption | Assumption |
*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.