Reviewer's report
Title:Risk of menstrual disorders among female registered nurse: a nationwide
retrospective cohort study in Taiwan
Version:2Date:22 March 2015
Reviewer: Chung-Yi Li
Reviewer's report:
This study explored, using medical claim data from Taiwan, whether nurses
experienced higher risks of menstrual cycle related disorders. A fairly large
number of registered nurses (RNs) (n=4,241) were included in this study
between 2001 and 2010. The frequency of ambulatory care visits for menstrual
disorders among RNs was compared to that of the year- and age-matched
controls. The multiple Cox regression model showed that RNs had significantly
lower hazard ratios of most of the end-points including amenorrhea, irregular
menstrual cycle, dysfunctional uterine bleeding, unspecified disorders of
menstruation and other abnormal bleeding from female genital tract, and
dysmenorrhea. The only end-point that was positively associated with nursing
occupation was premenstrual tension syndrome. As reported by the authors,
there was a significant interactive effect nursing occupation with calendar year for
several end-points. The strengths of this study included an interesting
hypothesis, a large sample size, and an important health care service issue.
Despite that, like many other medical claim-based studies, this Taiwanese study
is also subject to potential confounding due to lack of lifestyle and
socio-economical risk factors for menstrual disorders. Interpretations of the study
findings are also difficult, as no direct measures of work conditions of RNs. Some
other comments are listed below:
Major Compulsory Revisions
1. The authors indicated that “While the hazards of these five disorders in the
nurses were initially lower compared with the general population, they crossed
the zero hazard point at 385 days, 742 days, 1,932 days, 1,561 days, 133 days”.
It is not clear though what “cross the zero hazard point” was supposed to mean.
Does it mean that the hazard ratio was initially lower than 1 before a given time
point time, and then became greater than 1 after that point in time. If it is the
case, the underlying assumption of “proportionality” for the Cox regression model
is violated, and the hazard ratios shown in Table 1 were not valid.
2. The presentation of Table 1 is somewhat misleading. The regression
coefficient for the interaction term of nursing occupation with time is primarily
used to evaluate whether the “proportionality” assumption holds. If the interaction
term is significant statistically, further time-partitioning analyses are required.
Taking the exponential (#s) for the interaction terms does not make much sense.
3. On the second paragraph of Methods, the authors described that the index
date for the comparison group was assigned by a randomly selected ambulatory
visit within the matched index year. Does it indicate that women who had no
ambulatory care visits in a given year were with a zero chance of being selected
as control subjects? If the authors made such exclusion, the prevalence of
menstrual disorders would have been underestimated because healthier women
were less likely to be selected as controls.
Minor Essential Revisions
1. The authors used “time-dependent group” effect at several locations of the
manuscript. This is not a standard term in survival analysis.
Level of interest: An article whose findings are important to those with closely
related research interests
Quality of written English: Needs some language corrections before being
published
Statistical review: Yes, and I have assessed the statistics in my report.
Declaration of competing interests:
No competing interests to be seclared.