Title: Chlamydia Infection, PID, and Infertility: Further Evidence from a Case-control Study
in China
Version: 3 Date: 15 Dec 2021
Reviewer's report:
This is an interesting paper on the association between chlamydia (either previous or current),
PID and infertility. Results of the study indicate previous chlamydia infection was more
common among infertile women, although in regression analyses corrected for covariates
only a marginally effect was found. PID was associated with an increased risk of infertility.
Although interesting, this paper needs improvement of the results, specifically the way results
are presented. Furthermore, the discussion needs more elaboration on the results and discussion
on limitations and assumptions.
Specific comments include:
1. Is the chlamydia history variable a valid variable for this analyses with the current definition-
prevalence/numbers? In the study it was concluded that only a marginal association between
chlamydia infection history and infertility was found. Authors support this finding by possible
conflicting literature on the role of chlamydia and infertility, however the systematic review
which is used as a reference showed a high risk for infertility following infection. I think the
results which has been found in this study needs more elaboration. The prevalence of chlamydia
history in this study was very low (2.4%, 0.4% and 0.4%) especially when this is compared to
current chlamydia (NAAT, during pregnancy or fertility work up) which was 5.9%, 7.3% and
7.1%. This would suggest (and the authors mention this  briefly in the limitation section) that
chlamydia history in the study does not reflect the true chlamydia history. The result of this is
that many women were classified as chlamydia history negative, which might/probably resulted
in misclassifications. This might dilute the possible effect between chlamydia and infertility
significantly. Furthermore the numbers of previous chlamydia infection are extremely low (n =
6, 2, 2) How were proper analyses including adjustment for confounding done regarding these
low numbers? Please reflect on the validity of the chlamydia history variable (as well as the
current infection variable) and what impact that might have on the authors conclusion.
The future Pgp3 analyses will probably help addressing these issues.
2. The way the results are presented is unclear. In tables references categories would be helpful.
And in the results it is mentioned (line 160-162) that “PID diagnoses were associated with
an increased risk of infertility, as compared with both control groups”. The “as compared to
both control groups” is confusing. As it is written is suggest that the control groups are groups
without PID instead of infertility. Also the reference to “non-pregnant group” versus “pregnant-
group” causes confusion. As I understand correctly “non-pregnant group” contains the non-
pregnant control women and the casegroup. And the “pregnant-group” includes the pregnant
control women and the casegroup? Please add all the numbers in the table (chlamydia cases,
PID cases, etc).
Furthermore, it is not clear what association was tested and presented in table 3. As presented
in the table it suggest that primary and secondary infertility were the exposure and PID the
outcome. However in the manuscript is it suggested the other way around. Please clarify the
exposure and outcome and present this in a clear manner in the tables.
3. Despite that the authors only find a marginal effect of chlamydia and infertility, the
recommendations are entirely focused on chlamydia prevention (provide periodic testing in
high risk groups and exploration of chlamydia markers). Could the authors support these
recommendations. Why wouldn’t the authors focus on PID prevention instead of only and
specifically chlamydia?
4. It is not entirely clear  who the non-pregnant controls are. In line 93 the authors write: “we
selected those who sought health care in Obstetric/Gynecologic clinics”, however how do the
authors know these women are not cases? What was the reason they sought care in obstetric/
genecology clinics?
Minor comments
Overall
1. Please write the species name (Chlamydia trachomatis) italic.
Abstract
2. Please state clearly the outcome (infertility) versus exposure (chlamydia and PID).
3. Why was chosen for urine specimens instead of vaginal swabs? (lower sensitivity and
specificity).
4. Please add the reference category in the result section with the odds ratio’s.
Background
5. Line 52-53 the differences between the ‘overall infertility’ versus ‘women actively trying to
conceive’ is not clear to me. How is this different?
6. Line 64-66. Authors state that “True host and pathogen determinants underlying infertility
remain unknown”.  How is this issue solved using your design?
7. Line 73. Prevalence of chlamydia was 84.55 per 100 000, which was considered a considerable
disease burden. Please put this in perspective (for example compare to other countries or
disease).
Method
8. Line 86. Please clarify the definition of infertile women.
9. Line 89. How were tubal disorders diagnosed? And why include women with endocrine
disturbance? Do the authors expect an association with chlamydia or PID?
10. Line 91-92. Pregnant controls: were these women pregnant during study inclusion of have
they been pregnant previously?
11. Line 99-100. The question about alcohol drinking, was this based on drinking during
pregnancy or beforehand?
12. How was information about PID collected? What questions were asked?
13. Line 127. A potential risk factor of infertility was among other maternal age. But what ‘age’
did authors use for non-pregnant and infertile women?
14. What was the range in table 1 for monthly income based on?
Results
15. Line 144: Please add the comparison of dollars and RMB in table 1.
Discussion
16. Line 222: What effect on the results do you expect from the recall bias?
17. Line 223: The second limitation is based on chlamydia and PID together. Was this an original
aim of this study?