Author's response to reviews Title: Correlates of STI symptoms among female sex workers with trucker driver clients in two Mexican border towns Authors: Nadine E Chen (nadinechen@ucsd.edu) Steffanie A Strathdee (sstrathdee@ucsd.edu) Felipe J Uribe-Salas (fjusalas@gmail.com) Thomas L Patterson (tpatterson@ucsd.edu) Gudelia Rangel (grangel2009@gmail.com) Perth Rosen (perthrosen@gmail.com) Kimberly C Brouwer (kbrouwer@ucsd.edu) Version: 3 Date: 6 July 2012 Author's response to reviews: see over
UNIVERSITY OF CALIFORNIA, SAN DIEGO UCSD BERKELEY DAVIS IRVINE LOS ANGELES RIVERSIDE SAN DIEGO SAN FRANCISCO MERCED SANTA BARBARA SANTA CRUZ DIVISION OF GLOBAL PUBLIC HEALTH DEPARTMENT OF MEDICINE 9500 GILMAN DRIVE, MC-0507 LA JOLLA, CALIFORNIA 92093-0507 (858) 822-3018 FAX (858) 534-4642 July 6, 2012 Dear Dr. Jonathan Golub, We appreciated the helpful comments by the reviewer and the opportunity to revise and improve our manuscript (MS 4496234696825015), now entitled Correlates of STI symptoms among female sex workers with trucker driver clients in two Mexican border towns. We have worked diligently to revise the manuscript in accordance with the suggestions provided by the reviewer and have made the following corresponding changes. All authors have approved the following revisions. Below are our responses to the reviewer comments. 1) The title is overly lengthy and awkward We have changed the title to Correlates of STI symptoms among female sex workers with trucker driver clients in two Mexican border towns. 2) Analysis. Authors used backward stepwise regression analysis for variables with p<0.10 on univariate analysis. This is an interesting approach given their goal; stepwise regression certainly evaluates factors but is guided by automation rather than conceptual relevance. It enables factors to be kicked out of consideration without conceptual underpinning. At a phase of exploratory analysis such as this, many of the factors that were kicked out of the model are actually quite informative and it would be helpful to see adjusted analyses and understand the extent to which they are confounded by other factors. While stepwise regression is a valid approach, it may not be appropriate in this case. Numerous factors significant in univariate analysis did not even get entered into the final stepwise regression model, suggesting that significant and potentially valuable information is lost. For an exploration of factors relevant to STI symptoms, a more common and informative approach would be simple logistic regression adjusted for demographic confounders, in this case perhaps birthplace and interview location. I would urge the authors to consider this type of analysis as the field is just emerging, presenting basic adjusted associations of each of the potential factors of interest will represent an even more robust advance. We appreciate the insightful comment of the reviewer and have modified our analysis as suggested. We no longer present a stepwise regression model, but instead have adjusted for certain demographic variables only. We chose to control for age and study site. Although age was not significant on bivariate analysis, in previous studies it has been found to be an important correlate of HIV and STI risk behavior. Since study site and country of birth were collinear, we chose to only adjust for study site, which was the stronger association. 3. In a similar vein, much of the discussion is devoted to the alcohol and substance use pathways to STI/HIV risk while this is well done, it seems an oversimplification given the univariate findings
i.e., it is highly plausible that some of the other factors identified in univariate analyses are relevant to the causal pathway as mediators, confounders etc. After revision of analysis, we have also expanded on the discussion to include the additional factors that are now significant. 4. A point of confusion in the table is that the authors have presented % with and % without symptoms, while a more common and more informative approach would present % with symptoms across all exposure categories. In some cases such as workplace venue, all exposure categories are represented while in others such as has a manager/pimp, only those exposed are presented. Because the OR quantifies the relationship of the exposure (had a pimp vs didn t) with the outcome of STI symptoms, please present % STI symptoms across each exposure category with the use of additional rows as needed. We have now separated the data into two tables and clarified the referent values on the logistic regression table. 5. The referent category in many of the table rows is not clear and further information is needed. Please include a clearly described referent category for each exposure, and present the sample % and % infected for each. E.g., for education, for had clients from US or Guatamala in the past 6 months. We have added additional rows to both table 1 and table 2 clarify referent categories. 6. The table is overly lengthy and contains data that has not been well justified in the introduction, or discussed in the conclusion. Overall, a large number of factors are assessed relative to the sample size of only 200. Some elements could be dropped or perhaps be better handled in a demographics and sample description table for example what is the relevance of HIV testing behavior to the outcome or is it just a matter of describing the sample? Same with the gyn use data. We have separated the information into two tables with Table 1 consisting of descriptive data. We have also included a more detailed discussion in the Methods section of why we have included gynecologic visit data. 7. Please make a note in the methods section as to how missing data were handled. We have included a sentence in the Statistical analysis section to describe how missing data was handled. Missing data in the regression analysis was assigned the null value to allow for conservative estimates. 8. Please confirm in the methods section that analyses were conducted considering each individual risk factor as exposure, and its relationship to the outcome of STI symptoms. We have clarified the Statistical analysis portion of the Methods section by stating the above. 9. Measures- please provide further details how was pimp/manager assessed? Was any definition given? How about for forced sex? How was deported defined? We have clarified the above terms in the Methods section.
Pimp/manager was defined as anyone with whom the participant had to share a percentage of the money they receive from clients. Exposure to forced sex was defined as ever having been forced to have sex with someone through either physical or emotional pressure. Deportation was assessed by asking both the country from where and to where the individual was deported, thereby not limiting the results to only deportation from the U.S. 10. The limitations of the syndromic STI assessment are significant. They should be explicitly noted and referenced in the discussion. We agree that the use of self-reported STI symptoms is a major limitation of this paper and have further expanded on this point in the Discussion section. Self-reported STI symptoms can simultaneously overestimate STI prevalence (as certain symptoms can be attributed to non-sti related causes such as vaginal candidiasis) and underestimate prevalence (as symptoms of certain STI such as chlamydia, HIV, and syphilis may not be recognized until late in the disease course). 11. A minor point - several areas of awkward wording could be smoothed in the introduction e.g., especially by facilitating ; and increase STI/HIV risk to their female sex partners should be likely or plausibly or some indication of the conceptual conclusions being drawn. We have corrected the phrases pointed out above. 12. Presentation of results in the text is surprisingly sparse and should be expanded upon. Initially we had planned to submit this paper as a brief report and had, thus, limited our discussion. As it is submitted as an original article now, we have expanded on the results of the logistic regression analysis in the Results section 13. One of the other limitations of the study design is that it does not enable comparisons with FSWs who do not serve long distance truck drivers- so the extent to which the findings reflect FSWs in the region broadly, or something unique to this group, is unclear. Please expand on this. We do include a statement in the paragraph on limitations that this study was only performed on FSW servicing long-distance truck drivers and therefore may not be generalizable to all FSW in the region. We also include statistics from Mexican FSW in border cities who are not particularly serving long distance truck drivers, in order to provide a better understanding of how the studied FSW population compares to others 14. The authors make reference to the majority of sex work occurring in bars in both the abstract and conclusion of the text to support the need for interventions it is not clear what point is being made here and why. The relevance of the bars is not entirely clear, so please clarify. We have edited the discussion to clarify the importance of recognizing that the bar/cantina is the most common place of work. Alcohol use before/during sex work had the strongest association with STI symptoms in this study. The bar/cantina was the most common place to solicit clients, and this sex work environment presumably facilitates the likelihood of drinking alcohol before/during sex work. Therefore, the context of the alcohol use is important to recognize in providing an intervention that will improve safer sex practices while under the influence of alcohol. Increased condom
availability in bar settings is one example of an intervention that has increased safer sex practices in this environment. Thank you again for the opportunity to revise and resubmit this paper. Sincerely, Nadine Chen, MD, MPH