The Netherlands Chlamydia cohort study (NECCST) protocol to assess the risk of late complications following Chlamydia trachomatis infection in women

Table 1 Power calculation per outcome variable

	Expected cumulative prevalence in 2022		Number of samples needed for 80% power		Expected power with samples size N = 6600
	CT pos. History	CT neg. History	CT pos. History	CT neg. History	CT pos. History (n = 1700)	CT neg. History (n = 4900)
PID	7.7%	1.1%	63	195	>99%
Ectopic pregnancy	1.8%	0.9%	1415	4385	85%
Tubal factor subfertility	1.3%	0.2%	401	1241	>99%

Expected cumulative prevalence of PID, ectopic pregnancy and tubal factor subfertility was based on age distribution per outcome in the primary care database from the Netherlands Institute for Health Services Research (NIVEL – PCD) and the expected age distribution in NECCST in 2022 by CT status. Here from we derived the samples size, with 80% power, a significance level of 5% and a 1 to 3 exposed/unexposed ratio, per outcome in the CT-positive history and negative history group in 2022 (http://www.openepi.com). CT Chlamydia trachomatis infection, PID pelvic inflammatory disease

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