Systematic hypertension screening and follow-up attrition in Thai prisons: A cross-sectional study

Abstract

Background:

The living conditions in prisons may contribute to the development of hypertension. The effectiveness of hypertension screening depends on both the willingness of individuals to participate and the presence of security policies that support completion of screening. This study examined the prevalence of hypertension in prisons by using a systematic screening approach, identified associated risk factors, and examined the rate of loss to follow-up after hypertension screening.

Method:

This cross-sectional study was conducted in five prisons in Thailand and identified 11,290 individuals aged 18 years or older who met the inclusion criteria. Trained peer volunteers administered structured questionnaires and measured the participants’ blood pressure, height, and weight. Individuals who were screened as positive (score ⩾ 1) were further evaluated by nurses. Those with blood pressure readings of 140/90 mm Hg or higher were referred to hospitals for diagnostic confirmation and treatment. Logistic regression was used to analyze associated factors.

Results:

Of the 11,290 eligible individuals who were identified, 98.6% agreed to undergo hypertension screening. The overall prevalence of physician-confirmed hypertension was 4.0%. The prevalence was 5.8% among individuals aged 35 years or older, 6.4% among those with a family history of hypertension, and 3.5% among individuals incarcerated for 2 years or longer. Age 35 years or older (adjusted odds ratio (OR) = 3.21; 95% confidence interval (CI): 2.52–4.08) and a family history of hypertension (adjusted OR = 1.81; 95% CI: 1.46–2.23) were significantly associated with hypertension, whereas incarceration for 2 years or longer was protective (adjusted OR = 0.72; 95% CI: 0.59–0.88). The overall loss to follow-up rate was 6.7%.

Conclusion:

The prevalence of hypertension among incarcerated individuals in Thailand was low, suggesting that prison living conditions may not be as detrimental to health as commonly perceived. However, the observed loss to follow-up highlights the importance of strategies to improve engagement and ensure timely access to diagnostic and treatment services throughout the screening and diagnostic process.

Plain language summary

Hypertension screening and loss to follow-up in Thai prisons

Purpose of this study: People in prison may be at risk of developing high blood pressure (hypertension) because they have limited food options, and limited opportunities for exercise. Hypertension screening helps with early identification of the condition, ensuring individuals can receive timely treatment and care. For screening to be effective, people must complete the full screening process. This study determined how common hypertension is in Thai prisons, identified key risk factors, and determined how many people did not complete follow-up after initial screening.

How this study was done: Researchers collaborated with prison nurses and trained peer volunteers to collect data from individuals aged 18 years or older. Peer volunteers administered structured questionnaires and measured each participant’s blood pressure, height, and weight. Those who screened positive (score ⩾ 1) were further evaluated by prison nurses. Individuals with blood pressure readings of 140/90 mm Hg or higher were referred to hospitals for diagnostic confirmation and treatment.

Findings from this study: The study found that 4.0% of the participants had hypertension. The rate was higher among individuals aged 35 years or older (5.8%) and those with a family history of hypertension (6.4%). In contrast, the rate was lower among who had been in prisons for 2 years or longer (3.5%). The main factors linked to having hypertension were being aged 35 years or older and having a family history of hypertension. Staying in prison for 2 years or longer appeared to lower the likelihood of having hypertension. Overall, 6.7% of participants did not complete the full screening and diagnostic process.

Implications of study findings: This study provides insight into the relatively low prevalence of hypertension in Thai prisons. These results suggest that prison living conditions may not be as harmful to health as often perceived. However, the proportion of individuals lost to follow-up in this study indicates that stronger strategies are required to ensure individuals complete the full processes of screening and diagnosis.

Keywords

Hypertension screening prison prevalence

Introduction

Studies investigate the effect of prison living conditions on the development of hypertension among incarcerated individuals. Some studies suggest that the prison environment contributes to the development of hypertension because of various factors, such as limited food options and restricted opportunities for physical activity.^1–3 Several studies have reported a higher prevalence of hypertension among incarcerated individuals compared with among the general population.^4–7 By contrast, some studies have indicated that individuals in prisons abstaining from smoking, alcohol, and overeating may help prevent hypertension.^8,9 For example, in a study conducted in Australia, the prevalence of hypertension among incarcerated individuals aged 45–54 years was 24.4%, whereas it was 27.1% in the general population.¹⁰

Studies have identified several risk factors for hypertension among incarcerated individuals. Research has consistently reported that age >40 years, obesity, smoking, and a family history of hypertension are major risk factors for hypertension.^11,12 However, many relevant studies have been conducted in prison settings where individuals had substantial autonomy in determining their daily routines, with incarcerated individuals being allowed to smoke and eat more frequently, or where confinement periods were shorter. These factors may affect both the health behaviors and risk profiles of such individuals.

Systematic hypertension screening typically involves three steps: an initial risk assessment conducted using a structured questionnaire followed by blood pressure measurement. Individuals with readings of 140/90 mm Hg or higher undergo repeat measurements over 3 consecutive days. If blood pressure remains elevated, they are referred to a hospital for diagnostic confirmation and treatment. In prison settings, however, hypertension screening is voluntary, and many individuals remain unaware of their condition because they have no symptoms. Prison nurses are responsible for conducting these screenings, but they often encounter operational barriers.⁶ The hierarchical structure of prisons, which is often organized on the basis of sentence duration, can further complicate the process.² For example, transferring individuals serving life sentences to the healthcare unit for an asymptomatic condition such as hypertension is logistically difficult and may be hindered by restrictions related to security protocols.

Thailand houses 280,835 individuals across 168 prisons, and therefore, it is among the 15 countries with the largest incarcerated populations globally.¹³ Since 2016, Thailand has implemented a smoking ban in prisons and enforced a confinement period from 4.00 p.m. to 7.00 a.m., during which time individuals have no access to food. Routine reports from the Department of Corrections in Thailand indicate that 3.3% of incarcerated individuals have hypertension. This figure contrasts sharply with the rate observed in the general population, which are 23.2% for hypertension.^14,15 Whether this discrepancy reflects underdiagnosis or a true difference remains unclear, particularly because systematic screening data are generally lacking for this population.

Systematic hypertension screening is critical for accurately estimating prevalence, identifying at-risk individuals, and guiding the development of effective prevention and care strategies for incarcerated individuals. Therefore, the present study examined the prevalence of hypertension in Thai prisons by using systematic screening, identified associated factors, and determined the rate of loss to follow-up after hypertension screening.

Methods

Study design

This cross-sectional study was conducted in accordance with the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines.¹⁶ The study was conducted between September and December 2023 in five correctional institutions across Thailand. These institutions were purposively selected to represent diverse geographic regions, prison security levels, and population profiles. These institutions do not have on-site physicians; instead, individuals are transported to contracted hospitals for medical care. Correctional nurses are present in the facilities 24 h a day, with one nurse responsible for approximately 700 individuals.

Study population

All incarcerated individuals in the participating prisons were invited to participate in the study, and announcements were made through public address systems and peer volunteers. The inclusion criterion of this study was being at least 18 years of age, include Thai or non-Thai citizenship. In addition, individuals deemed too ill to participate were excluded. Participants were sequentially selected from each unit until all units had been included.

The primary outcome variable was hypertension. Participants were classified as having hypertension if they had a physician-confirmed diagnosis and systolic blood pressure of 140 mm Hg or higher or diastolic blood pressure of 90 mm Hg or higher.

The exposure variables included age 35 years or older, a body mass index (BMI) of 25 kg/m² or higher, and a family history of hypertension (parents or siblings). These variables were selected and categorized in accordance with the 2023 guidelines from the Thailand Ministry of Public Health.

Instruments

The hypertension screening involved assessment of three items: systolic blood pressure of 140 mm Hg or higher or diastolic blood pressure of 90 mm Hg or higher, a family history of hypertension, and age 35 years or older. Each item was assigned a score of 0 (No) or 1 (Yes), with a total score of 1 or higher indicating a risk of hypertension. For individuals identified as at risk, their blood pressure was measured over 3 consecutive days, and those with systolic blood pressure of 140 mm Hg or higher or diastolic blood pressure of 90 mm Hg or higher were referred to a hospital. Hypertension is defined as doctor diagnosis. Measuring blood pressure according to 2022 Thai Hypertension Society guidelines consist of (1) refrain from drinking coffee or tea at least 30 min before measuring blood pressure; (2) sit down on a chair and relax; (3) not talk before and during blood pressure measurement; (4) rest feet on the ground and not sit cross-legged.¹⁵ Digital sphygmomanometer was used to measure blood pressure.

Instrument validation

The content validity of the questionnaire was reviewed by four experts in cardiovascular disease and correctional health, representing Ministry of Public Health, Ministry of Justice, and Faculty of Nursing. The content validity index value was 1.0, indicating excellent validity.¹⁷ The questionnaire was also pilot-tested with 30 individuals in a prison to ensure its clarity and readability.

Data collection

The data collection process was completed in four steps in all investigated units. First, peer volunteers, each of whom was responsible for 50 individuals, invited eligible individuals to join the study. Second, after obtaining signed consent forms, the peer volunteers conducted interviews and measured participants’ blood pressure, height, and weight. Third, individuals with positive screening results (scores of ⩾1) underwent further evaluation for hypertension by nurses. Finally, individuals identified as at risk and who had blood pressure readings of 140/90 mm Hg or higher were referred to a hospital for diagnostic confirmation and treatment.

The study involved a large sample size and multiple data collectors. To ensure adherence to the study protocol and mitigate bias, correctional nurses on the research team completed a 1-day training session, with this followed by monthly meetings to monitor progress of data collection. Senior researchers conducted site visits at each study site to oversee operations and provide guidance. To reduce bias in collecting sensitive information face-to-face interviews were conducted in settings that ensured privacy and confidentiality, which minimized the likelihood of responses being overheard.

In consideration of the characteristics of the study population and the study’s emphasis on ensuring inclusivity, all individuals were invited to participate.

Statistical analysis

To test associations, age 35 years or older (yes/no), family history of hypertension (yes/no), and length of incarceration of 2 years or longer (yes/no) were selected as variables. To ensure that each variable is independent, multicollinearity among the variables was assessed using tolerance values. The tolerance for each variable was 1.0, indicating that no multicollinearity was present. Logistic regression was then used to assess factors associated with hypertension, and significance was determined by calculating 95% confidence intervals (CIs). Cases with missing data for any variables were excluded from the analysis. All analyses were conducted using STATA, version 14.2 (StataCorp, College Station, TX, USA), under a license held by the corresponding author.

Ethical consideration

This study was approved by the Research Ethics Review Committee for Research Involving Human Research Participants, Group I, Chulalongkorn University (COA No. 153/66). Written informed consent was obtained from all participants. All data were managed confidentially, and the participants’ identities were protected throughout the data collection process and in the presentation of the study findings.

Results

Sociodemographic characteristics of participants

Table 1 presents the characteristics of participants screened for hypertension. Of 11,290 eligible individuals, 11,134 participated in the screening, indicating a 98.6% response rate. Most participants were men (92.4%) and were relatively young, with an average age of 36.9 years. Drug trafficking was the most common reason for incarceration (68.1%). The median length of incarceration was 2.2 (interquartile range (IQR) = 1.0–4.0).

Table 1.

Characteristics of incarcerated individuals stratified by blood pressure levels.

	Blood pressure level (mm Hg)
Characteristics	Total (n = 11,134)	<140/90	⩾140 or 90
Sex^a (n, %)
Male	9897	8771 (88.6)	1126 (11.4)
Female	813	706 (86.8)	107 (13.2)
Nationality^a (n, %)
Thai	10197	9015 (88.4)	1182 (11.6)
Non-Thai	443	395 (89.2)	48 (10.8)
Universal healthcare^a (n, %)
Yes	9426	8226 (87.7)	1160 (12.3)
No	700	635 (90.7)	65 (9.3)
Age group^a (n, %)
≤34	4860	4534 (93.3)	326 (6.7)
35–44	3733	3265 (87.5)	468 (12.5)
45–54	1462	1203 (82.3)	259 (17.7)
≥55	653	473 (72.4)	180 (27.6)
Mean (SD)	36.9 (10.1)	36.2 (9.7)	41.9 (11.2)
BMI^a (kg/m²)
Mean (SD)	23.1 (3.4)	23.0 (3.4)	24.2 (3.8)
Family history of the disease^a (n, %)
Yes	2287	1950 (85.3)	337 (14.7)
No	8410	7514 (89.3)	896 (10.7)
Offense category (multiple responses) (n, %)
Drug trafficking	7286	6466 (88.7)	820 (11.3)
Robbery	965	863 (89.4)	102 (10.6)
Homicide	929	825 (88.8)	104 (11.2)
Other	1664	1444 (86.2)	222 (13.3)
Length of incarceration (years), median (IQR)	2.2 (1.0, 4.0)	2.2 (1.0, 4.0)	2.0 (0.9, 4.0)
>2	5,214	4,631 (88.8)	583 (11.2)
⩽2	4,977	4,385 (88.1)	592 (11.9)

BMI: body mass index; IQR: interquartile range.

Some variable data are missing.

Screening results

Positive screening results were slightly more frequent among women than among men (13.1% vs 11.3%). Individuals aged 55 years or older had a higher rate of positive results (27.6%) than did younger individuals. Individuals with positive results had a higher BMI (24.2 kg/m² for hypertension) and a greater prevalence of a family history of hypertension (14.7%). Individuals incarcerated for 2 years or longer had a slightly lower percentage of positive results (11.2%). (Table 1)

Figure 1 illustrates the hypertension screening process. Of the 11,134 participants screened, 411 were given a diagnosis of hypertension, with this including 319 newly identified cases and 92 previously known cases. The overall prevalence of hypertension was 4.0% (95% CI: 3.6–4.4). Among individuals aged 35 years or older with a family history of hypertension, the prevalence was 7.3% (95% CI: 7.0–7.7).

Figure 1.

Flowchart of hypertension screening among incarcerated individuals.

Factors associated with hypertension

In the multivariate model, participants aged 35 years or older were 3.37 times more likely to receive a diagnosis of hypertension (OR) = 3.37; 95% CI: 2.63–4.33), and those with a family history were 1.86 times more likely (adjusted OR = 1.86; 95% CI: 1.50–2.30) (Table 2). However, incarceration for 2 years or longer was a protective factor (adjusted OR = 0.77; 95% CI: 0.59–0.88).

Table 2.

Factors associated with physician-confirmed hypertension.

		Hypertension		Crude OR(95% CI)	Adjusted OR(95% CI)
	Total	Yes(n = 411)	No(n = 9964)
Factors	n	n (%)	n (%)
Age ≥35 years
Yes	5606	325 (5.8)	5281 (94.2)	3.35 (2.63–4.26)	3.37 (2.63–4.33)
No	4762	86 (1.8)	4676 (98.2)	Reference	Reference
Family history of hypertension
Yes	2188	140 (6.4)	2048 (93.6)	1.99 (1.62–2.46)	1.86 (1.50–2.30)
No	8170	271 (3.3)	7899 (96.7)	Reference	Reference
Length of incarceration (years)
>2	4954	173 (3.5)	4781 (96.5)	0.77 (0.63–0.94)	0.72 (0.59–0.88)
⩽2	4901	220 (4.5)	4681 (95.5)	Reference	Reference

OR: odds ratio; CI: confidence interval.

Lost to follow-up after hypertension screening

The overall attrition rate for follow-up was 6.7% (744 of 11,134 screened participants; (Figure 1). Among individuals identified at risk, particularly those aged 35 years or older, the attrition rate for screening was higher, at 11.9% (572 of 4824). Attrition occurred at two key points: 8.5% (408 of 4824) of at-risk individuals did not return for the required repeat measurements, and among those who did return, 25.3% (164 of 408) had no documented diagnostic outcome.

Discussion

The 4.0% prevalence of hypertension among incarcerated individuals in the present study was unexpectedly low. This may be related to protective factors within the prison environment, such as the prohibition of smoking and alcohol and the absence of frequent eating or access to fast food, which are well-known risk factors for hypertension.^18–20 In this setting, confinement from 4.00 p.m. to 7.00 a.m. without access to food created a dietary setting resembling that of intermittent fasting, which is associated with a decreased risk of cardiometabolic diseases. These findings represent the lowest prevalence reported in the literature and are consistent with the results of previous studies indicating a lower prevalence of hypertension among incarcerated individuals compared with among the general population.^11,21–23 Moreover, in this study, diagnoses were based on physician assessments and treatment initiation, which differs slightly from the methods employed in other studies, which have relied solely on laboratory results or blood pressure measurements. In addition, adult age is indicated by the mean age score in this study. The incidence of hypertension increases with age, with a lifetime risk of more than 90% for older people.²⁴

Several studies have reported a higher prevalence of hypertension among incarcerated individuals compared with among the general population.^12,24,25 This may be due to prison contexts. Three studies were conducted in facilities where smoking and alcohol consumption were not prohibited,¹² and this could have contributed to the studies observing a higher prevalence of hypertension than that reported in the current research. Also, using secondary data from external surveys,²⁵ which may have affected the study’s ability to accurately determine denominators. However, findings regarding the prevalence of hypertension among incarcerated individuals remain inconsistent, and further research is warranted to examine the prevalence of hypertension in diverse prison settings, with consideration of factors such as smoking policies, fasting duration, and access to junk food. Such studies could provide a clearer understanding of how and why specific prison practices affect the prevalence of hypertension.

Screening individuals in prison settings presents substantial challenges, particularly when screening is conducted for asymptomatic conditions such as hypertension. Requesting individuals who screen positive to visit the healthcare unit for repeated blood pressure measurements over 3 consecutive days is often difficult and referring those with elevated blood pressure to external hospitals for diagnostic confirmation is subject to several logistical barriers, especially in maximum-security facilities where movement is highly restricted. In the present study, 26.3% (324 of 1233) of individuals with systolic blood pressure of 140 mm Hg or higher or diastolic blood pressure of 90 mm Hg or higher had no documented diagnostic outcomes. This underscores the crucial role of the prison healthcare team, particularly correctional nurses, in facilitating timely diagnosis and access to medical care. Operational limitations in prison healthcare are not unique to Thailand. A study in Brazil reported that because of institutional regulations in prison settings, blood pressure screening involves a single measurement rather than the recommended three.⁶ Another study in the United States noted that incarcerated individuals often lack autonomy in accessing healthcare services, which can complicate screening and follow-up efforts.^3,26

The present study determined that both a family history of hypertension and age ⩾35 years were associated with hypertension. These findings are consistent with those of a study conducted in a central prison in Cameroon indicating that age >40 years and obesity were significantly associated with hypertension.¹² Similarly, a study in Zambian prisons reported a significant association between age >40 years and hypertension.¹¹ Notably, the present study identified the risk threshold to be at a younger age (35 years) than the more common threshold of 40 years reported in the literature. The higher prevalence of hypertension among younger individuals in this study aligns with guidelines from Thailand’s Ministry of Public Health, which recommend annual screenings for hypertension—including for individuals who are incarcerated—starting at the age of 35 years under the universal healthcare system in the country. Interestingly, longer durations of incarceration appeared to be protective against hypertension. This may be explained by restricted access to alcohol and cigarettes, as well as routine intermittent fasting practices (from 15.00 until 7.00 the following day).

The strength and limitation of the study

The findings of this study have crucial implications. As indicated by the current finding of a notably low prevalence of hypertension among incarcerated individuals, governments, healthcare professionals, and researchers must promote and maintain health-supportive practices, such as abstinence from smoking and alcohol consumption and adherence to intermittent fasting routines. In addition, ensuring timely diagnosis and treatment for incarcerated individuals with elevated blood pressure is essential, particularly when individuals are not referred to external hospitals. Interventions such as regular physician visits within prison settings or the integration of telehealth services should be considered as methods for improving access to care. Moreover, the large sample size in this study increases the precision and reliability of the results.

This study has some limitations that should be noted. First, the need to interview many incarcerated individuals within a short timeframe resulted in some missing demographic data. Second, reliance on self-reported information may have introduced social desirability bias, although measures were taken to encourage accurate responses without fear of punishment. Third, the number of female participants was relatively small, which reflects the demographic composition of the incarcerated population in Thailand. Finally, this study used cross-sectional designs that have limitation in establishing causal relationships. Hence, this study explains that findings reported associations at a single point in time.

Conclusion and recommendations

This study revealed that the prevalence of hypertension among incarcerated individuals in Thailand was lower than that reported in the literature, suggesting that their living conditions may offer protective effects against this condition. Although systematic screening prisons is feasible, effort must be expended to raise awareness of hypertension screening and to ensure timely diagnosis for individuals with elevated blood pressure levels. Being aged 35 years or older and having a family history of hypertension were both significantly associated with the condition. These factors can be incorporated into screening criteria, with additional annual check-ups provided for high-risk individuals, which can enable earlier diagnosis and appropriate intervention. Further study should consider other factors such as inmate adherence to treatment and medication/adherence, subjective factors (e.g. salt intake, physical activity), any factors uniquely relevant to the prison environment (e.g. quality of food/nutrition provided, overcrowding, access to routine healthcare) to add more detail information. Moreover, including biochemical measurements (e.g. blood sugar, cholesterol) to provide a more comprehensive picture of cardiovascular risk factors also recommended for further study. Prospective cohort studies in prison settings to observe the development of hypertension over time and truly establish predictive risk factors were suggested.

Footnotes

Acknowledgements

We acknowledge the contributions of the health volunteers and participants in this study. This manuscript was edited by Wallace Academic Editing.

ORCID iD

Sirinapha Jittimanee

Ethical consideration

This study received approval from the Research Ethics Review Committee, Chulalongkorn University (COA No. 153/66).

Author contributions

Ratsiri Thato: Conceptualization; Funding acquisition; Resources; Writing—review & editing.

Sirinapha Jittimanee: Conceptualization; Formal analysis; Investigation; Methodology; Project administration; Supervision; Validation; Writing—original draft; Writing—review & editing.

Nirut Rungsawasd: Conceptualization; Data curation; Resources; Writing—review & editing.

Woraphot Leerat: Data curation; Project administration; Writing—review & editing.

Haritphon Jaikham: Conceptualization; Data curation; Writing—review & editing.

Rohanee Etae: Conceptualization; Data curation; Writing—review & editing.

Chalita Prommart: Conceptualization; Data curation; Writing—review & editing.

Kotchakorn Tantivisan: Conceptualization; Data curation; Writing—review & editing.

Penpaktr Uthis: Conceptualization; Methodology; Writing—review & editing.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Health Systems Research Institution of Thailand (grant numbers 66-033, 2023) and the Center of Excellence for Enhancing Well-being in Vulnerable and Chronic Illness Populations, Chulalongkorn University, Thailand (grant number 1627/2568).

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data Availability Statement

Summary data are available upon request to the corresponding author.

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