A household cross-sectional study was carried out in Sistan and Baluchestan by the Zahedan University of Medical Sciences (ZAUMS) in 2017. Sistan and Baluchestan province, the widest province, is located in southeast of Iran with about 2.5 million people (
7). According to integrated health indicators categorization, the province is the poorest region in Iran (
12). The study population contained all households living in the province (about 587921 households in urban and rural area).
The study sample was calculated based on the following formula:

According to the previous study, α = 0.05, Z = 1.96, P = 14 (13), d = 0.02, and design effect = 2
In total, about 2400 households were selected for the study. According to the weight of the households in each city, in the province based on the 2011 census, which was done by statistical center of Iran, the samples were selected as following: Zahedan as the capital of province (816), Zabol (336), Iranshahr (264), Chabahar (312), Khash (192), Saravan (216), and Nikshahr (264), which were selected based on the their ration in urban and rural.The study sample was selected using stratified and cluster sampling in the urban and rural areas, respectively. Urban areas were divided into 5 districts (south, north, west, east, and center), each of which was considered to be a stratum. Sampling with the probability proportional to size was done in each stratum to determine its sample size. Having determined the city sample size, the sample size of each stratum was randomly selected using households’ ID numbers from the health centers. The questionnaires were completed by interviewing the heads or informed individuals of the households selected. If a researcher was unsuccessful after 2 tries in interviewing a household at a specific address, the next address was chosen as a replacement. In the rural area, each health house rural was considered to be a cluster and some of the health houses were selected by using systematic sampling. Since the number of households covered by each health house was different, sampling with the probability proportional to size was used to determine the sample size of the each health center. The selected sample size was equally divided among the wellness centers and the required data was gathered by interview of heads or informed individuals of the selected households. Researchers contacted the wellness centers to inform them of the number of households in their area that would take part in the interviews. The questionnaires were completed by interviewing the heads or informed individuals of the selected households.
The inclusion criteria in the study were tendency to participate in the study and attendance of the household head or informed person at the time of interview. The exclusion criteria included not remembering the household cost information.
Data was gathered using the household section of the questionnaire entitled World Health Survey (WHS), which was developed by WHO to evaluate the performance of health systems (
13). Kavosi et al. confirmed the validity and reliability of the questionnaire (
14). The WHS contains 2 main sections: the household questionnaire and the individual questionnaire. In this manuscript we report the results of the household questionnaire. It includes the following modules: ‘household roster’, ‘health intervention coverage’, ‘health insurance’, ‘health expenditure’, ‘indicators of permanent income’, and ‘health occupation’.
We determined a 1 month recall period for the household total expenditures, consumption rates, and outpatient medical care expenditures. Moreover, we determined a year recall period for the household expenditures and consumption of inpatient services. To calculate medical impoverishment, all expenditure variables were converted monthly. Moreover, the households expenditures were considered as an indicator of the households’ purchasing power as it has been mentioned in many previous studies, as well (
15).
2.1. Assessment of Medical Impoverishment
A non-poor household will be poor due to heath payments, if it becomes poor after paying for health services or staying longer in poverty. We measured medical impoverishment by using the following data: total household expenditure (EXP), household food expenses, family size to determine the equivalent family size, per-capita food expenditure, subsistence expenditure (SE), and OOP healthcare payments.
At first, family size and food expenses were converted into the equivalent family size and per-capita food expenditure, respectively. In order to obtain the household’s equivalent size, the real family size was powered by β [0.56], while in order to obtain the household’s per-capita food expenditure, the food expenses of the household were divided by the equivalent family size. Then, the ratio of food expenses to EXP was measured by dividing the food expenses by EXP and the households were ordered based on the obtained numbers. The poverty line was calculated as the household’s average equivalent food expenditure, which was from the 45 to 55 percentiles of the food expenditure to EXP. This number was considered to be the food poverty line in the research community. By taking this poverty line into account, each household’s SE was measured according to the following formula:

In order to show the effect of impoverishment due to health payments, the variable impoorh was made. If the EXP of a household was equal or exceed the SE but was less than the net SE after OOP healthcare payments, that household is incurring medical impoverishment, as defined below:

2.2. Determinants of Medical Impoverishment
Logistic regression model was employed to predict the likelihood of facing medical impoverishment and to calculate the odds ratios (OR) using the model coefficients. Besides, P-values less than 0.05 were considered as statistically significant. The proportion of households facing medical impoverishment is a dependent variable. The independent variables were gender, job, and educational status of the head of the household, location of residence, number of the household members, household economic status, number of members under the age 5 and over the age 65, basic and supplementary insurance status of the head of the household, use of dental services and inpatient medical services, OOP payment for medicine, physiotherapy, diagnosis services, existence of individual(s) in the household that require chronic medical care, and having a patient in hospital.
All the statistical analyses were performed using the SPSS statistical software (version 22. Armonk, NY: IBM Corp).
In the study, an informed consent form was completed by each participant and the authors have kept the information confidential.