Although Korea has observed substantial improvements to the quality of mental health care services, national statistics suggest further efforts are needed to improve mental health care efficacy. The average length of stay (LOS) for psychiatric patients has increased in Korea over the last 10 years [1]. For instance, the average LOS in 2017 was 168 days, five times higher than the average observed within other Organization for Economic Cooperation and Development (OECD) countries (29.4 days). In 2011, the psychiatric readmission rate for schizophrenia in Korea was 19.4%, the second highest among all OECD countries [2]. Additionally, incidence rates of psychiatric emergency treatment and the use of hypnotics among patients (diagnosed with mental and behavioral disorders due to use of alcohol; schizophrenia, schizotypal and delusional disorders; and mood disorders) showed an increasing trend between 2015 to 2019 in Korea [3, 4]. Psychiatric emergency treatment (reimbursed by Korean National Health Insurance) includes assessing patients' psychiatric symptoms through emergent interviews, applying seclusion and restraints in order to prevent harmful behaviors to themselves or others, and ongoing assessments until stabilization [5]. Thus, investigating factors associated with LOS, readmissions, and treatment amount, including pharmacological treatment, is worthwhile.

Nurse staffing is an important factor that influences patient safety and recovery. Staffing issues are also important for health policy provisions that establish domestic legal standards, which ensure compliance at each hospital. In Korea, standards for nurse staffing in psychiatric wards are stipulated in the Enforcement Rule of the Act on the Improvement of Mental Health and the Support for Welfare Services for Mental Patients, Article 11, Paragraph 2 [6]. According to this Article, one nurse is assigned to 13 psychiatric inpatients, and half of the registered nurse capacity can be substituted by nurse assistants [6]. This standard is considered insufficient to ensure adequate nurse staffing in comparison with standards elsewhere. For example, in the US state of California licensed nurse-to-patient ratio during one shift for acute psychiatric wards is 1:6 [7], while in Japan the employed nurse-to-inpatient ratios for the number of psychiatric beds are 1:3 or 1:4 [8, 9]. Therefore, it is essential to identify associations between nurse staffing and health outcomes among psychiatric inpatients in order to review and revise legal standards in Korea.

According to previous studies, higher nurse staffing levels were linked to reduced LOS [10, 11], readmission rates [12, 13], and mortality among medical or surgical patients [14]. Higher nurse staffing levels were also associated with fewer medical error incidences, pressure ulcers, and patient falls with injuries [15]. Several studies also examined associations between nurse staffing and health outcomes among psychiatric inpatients. Some reported that a greater number of psychiatric nurses were related to decreased LOS [16, 17], and more total nurses were associated with fewer psychiatric readmissions within 30 days [18]. Other studies investigated the relationship between nurse staffing and seclusion/restraint use, observing inconsistent results [19, 20, 21, 22]. However, the amount of research conducted in this area is limited, especially in Korea. Moreover, the research conducted to date focuses on the influence of nurse staffing on general health outcomes among psychiatric inpatients considering a few health outcome variables. Therefore, this issue warrants further attention.

Health outcomes related to psychiatric nursing care services can include LOS, readmissions, use of seclusion/restraints, use of injected psycholeptics for chemical restraint, and hypnotics use. Improving the quality of care services with adequate nurse staffing was previously shown to help prevent unnecessary extension of hospitalizations and readmissions [23, 24]. Research suggested that mental health nurses played an important role in reducing restrictive interventions, such as seclusion and mechanical or chemical restraint, by providing therapeutic interventions to aid patients who are at risk of harming themselves or others [25, 26]. Furthermore, sleep disturbances were prevalent among psychiatric patients [27], and nurses were in a prime position to help patients achieve adequate sleep by providing environmental, non-pharmacological, and pharmacological interventions [28, 29]. Thus, it is necessary to investigate the relationship between nurse staffing and these important patient outcomes within psychiatric wards. The present study investigated the association between nurse staffing and health outcomes of psychiatric inpatients (LOS, readmission, psychiatric emergency treatment, use of injected psycholeptics, and hypnotics use) using National Health Insurance claims data from Korea.

The median number of inpatients per RN, adjusted inpatients per RN, and adjusted inpatients per nursing staff in psychiatric wards were 16.77, 18.01, and 10.51, respectively. In terms of hospital type, only psychiatric wards in tertiary and general hospitals met the Korean legal standards for the RN-to-inpatient ratio (1.48 and 3.37, respectively) and RN-to-adjusted inpatient ratio (3.97 and 6.14, respectively), while nursing staff-to-adjusted inpatient ratio for all types of hospitals met appropriate expectations. However, when comparing the current nursing staff-to-adjusted inpatient ratio with Japanese legal standards for the number of psychiatric beds [8, 9], only tertiary hospitals showed a similar staffing level (3.56). The U.S. nurse staffing standard (i.e., California legal standard) is expressed as the licensed nurse-to-patient ratio during one shift [7]. Kim et al. [17] suggested that the U.S. standard of 1:6 for acute psychiatric wards can be converted to an employed nurse-to-patient ratio (a ratio which includes administrative nurses, nurses working with outpatients, and off-duty nurses) of 1:1.25 in Korea. Thus, when compared to the U.S., all types of hospitals had insufficient nursing staff in psychiatric wards. Additionally, the number of nurses included in the present study counted ward managers and nurses working in psychiatric outpatient and daycare wards, not just psychiatric inpatient wards. Thus, in practice, the number of inpatients per RN during one shift may actually be higher than what was reported in the present study.

According to the Korean legal standard for psychiatric wards, the number of employed nurses (not equal to the number of nurses during one shift) is used to calculate the nurse-to-patient ratio, and nurses can be replaced by nurse assistants within half of the registered nurse capacity [6]. This condition might exacerbate the qualitative deviations in the nurse staffing levels by hospital type and shift in clinical practice. For instance, it is possible to have only nursing assistants assigned to a shift within hospitals with low nurse staffing levels [17]. Therefore, the nurse-to-patient ratio per shift, which is presented in detail according to the type of hospital, should be prepared in domestic legal standard.

Based on the multilevel modeling analyses, nurse staffing variables were significantly associated with five health outcomes among inpatients. First, more inpatients per RN, adjusted inpatients per RN, and adjusted inpatients per nursing staff were associated with longer LOS. This result is similar to previous findings reporting significant LOS differences relative to nurse-to-patient ratios in psychiatric units. Specifically, fewer patients per nurse was previously related to a reduced LOS [17], and having more nurses in general hospital psychiatric units was associated with shorter LOS [16]. However, it should be noted that measures used to determine nurse staffing levels in previous studies differ from what was used in the current study [16, 17]. To help psychiatric inpatients return to the community, it is important to reduce hospitalization length by providing improved quality of care and promoting stabilization and recovery [17]. Nurses' ongoing observations and assessments, recognition, interpretations of clinical data, and decision-making were found an important factor for patient recovery [24, 41]. Sufficient nurse staffing was previously shown to enable effective surveillance and interventions, contributing to recovery and reduced LOS [24, 42].

The second significant finding was that more inpatients per RN, adjusted inpatients per RN, and adjusted inpatients per nursing staff were also associated with a higher risk of readmission within 30 days. The current results are similar to a previous finding, which reported that the number of nurses had an inverse association with readmission risk within 30 days [18]. Mental health nurses need to develop patient-centered recovery strategies and care plans during the transition from the hospital back into the community, while providing nursing support for patients' recovery through medication management, patient education, discharge plans, interpersonal effectiveness, and practical daily living [23, 43]. In particular, early non-adherence to medication after discharge was a crucial factor influencing readmission [44]. Hence, in order to reduce readmissions in patients with mental illnesses, hospitals should make effort to have a sufficient nurse staffing.

Third, more adjusted inpatients per RN and adjusted inpatients per nursing staff were also associated with less psychiatric emergency treatment, which corresponded to a diminished need to use seclusion/restraints. That is, a lower nurse staffing level was associated with a lower risk of psychiatric emergency treatment. Previous studies investigating the relationship between nurse staffing and seclusion or restraint use reported inconsistent results. Specifically, while De Lacy [22] reported that more nursing staff within psychiatric wards was related to fewer instances of using seclusion and restraints, Janssen et al. [19] found these variables to be unrelated. Fukasawa et al. [21], on the other hand, reported that more nurses per bed were associated with an increased use of seclusion and mechanical restraints. In contrast to common expectations regarding nurses' roles and abilities in reducing the use of restraints by employing knowledge and skills to meet patient needs and engage as active partners in the treatment process [25], the current study observed that lower nurse staffing levels were associated with less psychiatric emergency treatment. This might be due to insufficient adjustments to understanding patient severity regarding the nature of administrative data. It could also be that patients who were anticipated to need coercive measures, such as seclusion and restraints, tended to be admitted to wards that were assigned more nurses [21]. In general, hospitals with patients in less severe conditions tended to have lower nurse staffing levels [45]. Thus, hospitals with patients exhibiting a high severity of acute symptoms may employ more nurses to enable proper treatment and coping.

In the present study, we examined only the raw nurse staffing level without examining the qualitative characteristics of nurses' educational and work experience. Previous studies reported that nurse staffing, nursing work environments, and nurse education were all associated with patient outcomes [19, 46]. Thus, the development of a measurement tool that can properly adjust for inpatient severity is necessary and further studies are needed to investigate the relationship between nurse staffing and the use of seclusion and restraints.

The fourth outcome of the present study found that the relationship between the nurse staffing level and injected psycholeptics for chemical restraint was not significant. Although the influence of nurse staffing on the use of chemical restraints has rarely been investigated, it is expected that mental health nurses could help patients stabilize and reduce the use of chemical restraints [22, 25]. However, we found that the nurse staffing level was not associated with use of injected psycholeptics. The current result could be interpreted in two ways. First, patient characteristics, such as diagnosis and symptom severity, may have greater effects on the use of injected psycholeptics than do nurse staffing levels. Alternatively, the nurse staffing level may affect the use of injected psycholeptics, but significant relationship was not confirmed when not accounting for the qualitative aspects of the nursing staff present. Further studies should consider proper adjustments based on patient severity and measurement of both quantitative and qualitative aspects of nurse staffing levels. Moreover, we suggest that a longitudinal study is needed to analyze the influence of the nurse staffing level on changes to the number of mechanical and chemical restraints simultaneously employed.

Fifth, more adjusted inpatients per RN and adjusted inpatients per nursing staff were also associated with an increased use of hypnotics. Nurses were reported to often be the first to recognize sleep-related problems in patients helping them manage sleep disturbances through behavioral modifications and stimulus control [29, 47]. A previous study found that a sleep enhancement care as the nursing intervention for psychiatric inpatients was effective in improving subjective sleep quality, although patient consumption of psycho-active medications did not change between admission and discharge [47]. According to the Nursing Intervention Classifications, nurses may provide sleep enhancement care by assessing patterns of sleep and activities, sleep hygiene education, promoting daytime activities, and applying progressive muscle relaxation or massage [48]. Therefore, sufficient staffing may allow nurses to provide tailored sleep enhancement care to patient possibly contributing to the prevention the excessive use of hypnotics.

In the present study, the relationship between nurse staffing and patient outcomes showed similar patterns regardless of the type of nurse staffing (i.e., RN vs. nursing staff). These results might suggest that patient outcomes in this study were not closely related to nursing interventions for conditions such as urinary tract infection and pneumonia [49, 50]. Therefore, future research is needed to investigate the differences in the influences of number of patients per RN and number of patients per nursing staff on nursing-sensitive outcomes in psychiatric wards. Further, research is needed to identify the optimal nurse staffing levels and nursing skill mix to improve psychiatric health outcomes.

This study examined the relationship between nurse staffing level and psychiatric health outcomes based on PCDM thus contributing to the body of knowledge on the importance of nurse staffing in meeting patient needs and achieving positive health outcomes. The current findings also provided evidence that can be used for reviewing domestic legal standards and establishing policies on the nurse staffing in psychiatric wards.

A few study limitations should be noted. First, hospitals that did not submit estimation reports on the differentiating policies for hospitalization fees in medical aid psychiatric departments were not included in this study. Out of the initial 710 institutions that claimed reimbursement for psychiatric inpatient services to inpatients with primary diagnoses of F10.x, F2.x, or F3.x in 2016, approximately 200 institutions did not include staffing information. Therefore, one might argue that the present study did not provide an accurate overview of the nurse staffing ratios within the Korean healthcare system. Second, the nurse staffing level reported in the current study may not be completely accurate, as the number of RNs sampled included administrative nurses (i.e., outpatient and daycare nurses and ward managers). Thus, the nurse staffing level presented in this study might not be the same as the actual level in practice. Third, adjusting for patient severity while using National Health Insurance claims data was insufficient given that we were limited in our access to the available information and variables. Fourth, some hospitalizations and readmissions due primarily to the lack of family and social support systems may have been included, because it is not possible to screen hospitalization episodes using administrative data. These cases are likely to result in longer LOS or more readmissions. Fifth, since the severity and characteristics of psychiatric inpatients and the function of hospitals may differ depending on the type of hospital, the results of this study may not be generalized to all types of psychiatric hospitals. Hence, future studies should consider hospital type and examine hospitals with similar functions and having patients with comparable characteristics and symptom severity in their psychiatric wards. Sixth, non-benefit medications were not included in the National Health Insurance claims data.

This study investigated the association between nurse staffing level and psychiatric health outcomes based on PCDM. The current study found that nurse staffing levels were associated with health outcomes of psychiatric inpatients. Particularly, a lower nurse staffing level was related to a longer LOS, higher risk of readmission within 30 days, and increased hypnotics use. To improve the health outcomes of psychiatric inpatients, adjustments need to be made to increase the nurse staffing level in psychiatric wards as prescribed in the existing national law by establishing detailed standards according to shift work and the type of hospital. A review of domestic legal standards is needed to improve psychiatric nurse staffing levels and enact policies for optimizing nurse staffing in Korea.

Supplement 1

Research framework for predicting psychiatric patient outcomes

Supplement 2

Distribution of Patient Outcomes (N=70,136)

Supplement 3

Results of Multilevel Analyses of Model A Predicting Health Outcomes

Supplement 4

Results of Multilevel Analyses of Model B Predicting Health Outcomes