This study was conducted at an ANC clinic of King Chulalongkorn Memorial Hospital, Bangkok, Thailand. It was approved by the Institutional Review Board of the Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand (IRB No.028/61) and registered on the Thai Clinical Trials Registry (TCTR20180307003).
Estimating Initial WTP Prices
In March 2018, 50 conveniently sampled husbands of pregnant women were asked how much they were willing to pay for HIV and syphilis screening tests. They were not aware that the hospital prices of HIV and syphilis tests were US$4.5 or 140 Thai Baht (US$1 = 31 Thai Baht) and US$3.2 or 100 Thai Baht, respectively. The survey revealed the mean starting WTP prices of US$14.5 (SD 281.7) for HIV and US$9.7 (SD 270.8) for syphilis screening tests. These mean prices were chosen as the initial WTP prices with stepping up or down by ¼SD (US$2.2).
The questionnaire contained three sections. The first section obtained demographic information including age, educational level, and history of having a child. The second section obtained the history of HIV and/or syphilis screening tests and knowledge of HIV prevention using the United Nations General Assembly Special Session on HIV/AIDS (UNGASS) score (0-3, low; 3-4, moderate; 5, high). Based on the evidence from literature, the age16, income16,17, educational level14,16,19,20, history of having a child16, history of STI screen tests14,16,22, and knowledge of HIV prevention16 were considered potential factors affecting WTP and willingness to test for STIs. Other potential determinants such as the number of ANC visits20 were excluded because they were beyond the scope of this survey.
The third section asked about the willingness to test and the WTP for HIV and syphilis screening tests, using two contingency valuation methods: payment scale and bidding. For the payment scale method, the range of WTP price for HIV screening test was US$3.2 to US$25.8 (100 to 800 Thai Baht) whereas that of syphilis was US$2.5 to US$18.7 (77.5 to 579.7 Thai Baht). The husbands were informed of all available WTP options before choosing the price that they were willing to pay for the tests. For the bidding method, the participants were asked a series of Yes/No questions about their WTP specific prices for HIV and syphilis tests. First, they were asked whether they were willing to pay US$14.5 (450 Thai Baht) for HIV and US$9.7 (300 Thai Baht) for syphilis screening tests. Depending on their responses, the interviewer presented the US$2.2 (70 Thai Baht) higher bid for respondents answering “Yes” and the US$2.2 (70 Thai Baht) lower bid for respondents answering “No”. The question was repeated for the maximum of five and four rounds for HIV and syphilis tests, respectively (Figure 1 and Figure 2).
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At the first ANC visit, all husbands accompanying their pregnant wives were invited to participate in the study and informed about the ANC service process. After obtaining the written informed consent from the participants, they were allocated into two groups of WTP methods—payment scale or bidding—randomly assigned by date (payment scale on odds day and bidding on even day). We used face-to-face interviews to obtain information in the questionnaire as described above. Then, the husbands received blood drawn for HIV and syphilis screening tests; free of charge for HIV and US$3.2 (100 Thai Baht) for an optional syphilis screening test.
Sample size estimation
The sample size of this study was calculated by using the finite population mean formula.13 We estimated the sample size by using the value gained from the pilot study in March 2018, population size (N) was 188 (number of accompanying husband), standard deviation (σ) 281.7 (Thai Baht), error (d) 0.1 and alpha (α) 0.5; at least 188 subjects were anticipated. To elicit WTP for STI screening tests using the two methods, 200 subjects were enrolled in the study by convenience sampling and randomly separated into two equal method groups.
Interval variables including the WTP prices were analyzed by using the median (interquartile range, IQR) whereas categorical variables including the number of husbands who were willing to receive STI screening tests were analyzed by using percentages. The odds ratio was used to assess the association between an independent categorical variable and the willingness to get STI tests. Multivariate logistic regression analysis was used to determine factors associated with willingness to test for the STIs. SPSS version 22 was used to analyze the data and statistical significance was set at P<0.05.