The Opiate Dosage Adequacy Scale (ODAS) is a semi-structured clinical interview designed for estimating the adequacy of methadone or buprenorphine doses for patients with opioid use disorder (OUD) (1, 2). ODAS’s effectiveness relies on its proven ability to estimate the adequacy of a given dose based on multiple clinical parameters including continuous heroin use, cravings, narcotic blockage, objective and subjective withdrawal symptoms, and signs of overmedication (3, 4). The ODAS was originally developed and validated with Spanish-speaking European patients receiving medication for OUD (5, 6). The original validation demonstrated good reliability and construct validity, with Cronbach’s alpha coefficients ranging between 0.76 (methadone patients) and 0.92 (buprenorphine patients), with coefficients for each of the domains ranging between 0.81 and 0.93 (1, 2). ODAS scores were correlated with clinician assessments of treatment sufficiency and with patient self-reported outcomes, underscoring its clinical relevance in opioid maintenance therapy. Subsequent studies confirmed that ODAS-based dose classification is aligned with longitudinal treatment outcomes, reinforcing its clinical applicability in opioid maintenance therapy, although no Cronbach’s alpha was reported (4). Beyond European settings, the ODAS has been validated for cross-cultural use. A Persian adaptation of the ODAS demonstrated comparable psychometric properties, confirming its internal consistency (Cronbach’s α > 0.70) and convergent validity with other addiction severity measures (e.g., the Severity of Dependence Scale, SDS) (7). These findings suggest that the ODAS is a robust and adaptable instrument, capable of assessing opioid dose adequacy across diverse treatment populations. However, its applicability to other non-European Spanish-speaking populations remains unstudied.

Given the complexities of OUD pharmacotherapy, the ODAS provides a standardized approach for dose assessment that integrates both clinical symptoms and patient-reported outcomes. Previous validation studies have demonstrated its strong reliability and validity for determining whether a patient’s medication dose is sufficient to maintain clinical stabilization (2, 4). Despite the growing burden of opioid use disorder among Hispanic and Spanish-speaking individuals in the United States and across Latin America, few validated tools exist for guiding opioid dose adjustments in a linguistically and culturally relevant manner (8, 9). Given the clinical importance of ensuring appropriate buprenorphine dosing in opioid maintenance therapy, there is a critical need for validated instruments that are not only clinically sound but also tailored to the language and cultural context of diverse populations. Expanding the validation of the ODAS beyond its European origins to include Spanish-speaking populations in the Americas can contribute significantly to addressing treatment disparities and supporting individualized care strategies in these communities. Importantly, psychometric properties of clinical tools often vary across cultural and clinical contexts; hence, the ODAS measures developed in one population may not fully capture treatment adequacy in another. This gap is particularly important for Puerto Rican and other Hispanic subgroups who are disproportionally affected by opioid use disorder and who may experience distinct patterns of treatment access and retention. Validating the ODAS in these populations is therefore critical to ensure its reliability, validity, and applicability in these clinical settings, and to promote equitable assessments of treatment adequacy.

In this study, we aim to evaluate further the ODAS’s psychometric properties and its applicability to Spanish-speaking patients outside of Europe receiving buprenorphine OUD therapy evaluating the scale’s internal consistency (Cronbach’s alpha coefficient) and convergent associations with clinical stabilization variables such as severity of dependence (using the Severity of Dependence Scale [SDS]) and illicit drug use (2). We also aimed to examine the associations with patients’ buprenorphine levels (1).

The institutional review board of the University of Puerto Rico Medical Sciences Campus approved the study before the recruitment started (#B1080320).

The results reported herein support the use of the Opiate Dose Adequacy Scale (ODAS) in Spanish-speaking, non-European individuals maintained on buprenorphine. Our study subjects closely matched the demographic profiles, including age, sex, education, and employment distributions, of those in previously reported ODAS convergent validity studies conducted in Europe (2).

The dose-adequacy distribution of recruited subjects (54.9% were adequately dosed; mean total score: 29.79 ± 0.61) was lower than that reported for Europeans (73.3%; mean total score: 28.58 ± 2.55); more patients had an inadequate dose in this study (2). Our results demonstrated the ODAS’s capability to discriminate between adequately and inadequately dosed individuals, as evidenced by statistically different ODAS total scores between groups. Item 4, objective withdrawal symptoms, had the lowest scores in both groups (which was also the case for the European subjects). This item can then be an indicator of dose inadequacy (subtotal score < 4) in this population; a major cause for dose inadequacy. The optimal ODAS cut-off score for this group has been identified as 29. Patients with scores greater than 29 are significantly more likely to receive an adequate dose, while those with scores less than 29 tend to be inadequately dosed. Further studies are essential to validate this cut-off value for a clear classification of dose adequacy within this population, particularly focusing on patients’ objective withdrawal symptoms (item 4), as these symptoms can significantly impact dose adequacy. These results, consistent with previous findings, have demonstrated that the ODAS can be used to determine dose adequacy in a group of non-European Spanish-speaking patients with OUD and, as such, can be used as a clinical assessment for providers to identify patients that require a dose adjustment and improve maintenance treatment effectiveness (1, 2, 4). Further implementation studies should be conducted to test this intervention properly.

While the internal consistency of the ODAS in this sample was moderate (Cronbach’s α = 0.615), we acknowledge that Cronbach’s alpha has certain limitations when applied to Likert-type scales. Specifically, it assumes tau-equivalence and continuous measurement, which may not fully reflect the reliability of ordinal, symptom-based responses. Future analyses should consider alternative indices such as ordinal alpha or McDonald’s omega, which are better suited for ordinal data and can provide more robust estimates of internal consistency (17). Additionally, inter-rater reliability (IRR) was not assessed in this study because all interviews were conducted by a single trained researcher. While this minimized variability in scoring, it also limits our ability to assess consistency across different raters. Future studies should evaluate IRR through standardized methods (e.g., intraclass correlation or Cohen’s kappa), particularly given the semi-structured format of the ODAS, to ensure generalizability across clinical settings.

One important finding of this study was that dose adequacy was inversely proportional to the prescribed daily dose of buprenorphine; inadequately dosed patients had higher prescribed doses than those with adequate doses. Inadequately dosed patients in this study, having mean plasma values of 2.44 ng/mL (median 1.60), can explain their low treatment effectiveness (dose inadequacy) even though having higher doses than the adequately dosed group. Plasma levels of ≥ 3 ng/mL are required for a higher (> 80%) saturation of the opioid receptors and, therefore, treatment effectiveness (18). Patients in the adequate dose group had average plasma levels of 3.36 ng/mL. The reason for patients with inadequate doses having lower plasma levels than patients with adequate doses, even though taking relatively higher doses, may be explained by a combination of patient compliance regarding dosing schedules and pharmacological characteristics in this group.

To further explore the observed results between buprenorphine dose, plasma levels, and clinical dose adequacy, we examined the plasma-to-dose ratio (ng/mL per mg of buprenorphine) across different dose splitting frequencies. As illustrated in Figure 1, mean plasma-to-dose ratios varied across dosing patterns, with high inter-individual variability evident within each group. When stratified by dose adequacy, the plot shows that adequately dosed participants (green) generally achieved higher plasma exposure efficiency than inadequately dosed participants (red), even when using similar or lower total daily doses. This suggests that higher prescribed doses in the inadequate group did not translate into proportionally higher plasma concentrations, potentially due to suboptimal adherence, metabolic differences, or pharmacokinetic fluctuations associated with irregular dosing schedules.

Notably, the majority of participants in this sample reported dividing their daily buprenorphine dose into two to six doses per day, often following asymmetric or personalized patterns. These practices, while common, may lead to variability in trough plasma levels, particularly in the inadequate group. The data underscore the importance of considering pharmacokinetic efficiency, not just total dose, in assessing and optimizing buprenorphine treatment. Incorporating both clinical assessment tools like ODAS and objective pharmacokinetic data may offer a more complete understanding of dose adequacy in diverse treatment populations. Evidenced-based dosing schemes are an alternative, even more so in the current fentanyl era within the opioid crisis where higher (>24 mg) and varied dosing schemes are proposed as treatment effectiveness increase strategies (19, 20).

Building on these findings, our team has previously developed a population pharmacokinetic (popPK) model tailored to Spanish-speaking individuals with opioid use disorder in Puerto Rico (20). This model quantitatively characterizes buprenorphine exposure and predicts steady-state plasma levels under different dosing regimens. Notably, the model suggests that standard daily dosing may be insufficient to maintain therapeutic plasma levels (>3 ng/mL) in a significant proportion of patients, particularly those with altered absorption or metabolism. In contrast, higher total daily doses administered in divided dosing schedules (e.g., 8 mg TID or 16 mg BID) were shown to better sustain receptor saturation and minimize trough concentrations.

These findings support the use of the ODAS as a practical and psychometrically sound tool for assessing buprenorphine dose adequacy in Spanish-speaking populations outside of Europe. The scale’s ability to capture multidimensional markers of clinical stability, combined with pharmacokinetic data, highlights the complexity of dose-response relationships in opioid use disorder treatment. Notably, participants classified as inadequately dosed exhibited lower plasma-to-dose ratios despite receiving higher daily doses, suggesting suboptimal exposure efficiency. Divided or asymmetric dosing patterns, commonly observed in this sample, may contribute to these discrepancies. To address this, the integration of a population pharmacokinetic (popPK) model with ODAS-based assessment offers a translational approach for identifying patients who may benefit from optimized dosing schedules. This model may be used to guide individualized dose adjustments, aiming to maintain therapeutic plasma levels and improve treatment outcomes. In the meantime, providers can use ODAS scores in conjunction with assessments of objective withdrawal symptoms and patient-reported dosing behaviors to inform clinical decisions or reinforce buprenorphine adherence education. Ultimately, combining clinical insight with pharmacokinetic guidance represents a promising strategy to enhance treatment precision and effectiveness among Spanish-speaking individuals with opioid use disorder.

The psychometric evaluation of the ODAS in this Hispanic sample lays the groundwork for its broader application across Spanish-speaking populations in the Americas. As opioid use disorder continues to disproportionately affect underserved and linguistically diverse communities in the United States and Latin America, the availability of culturally responsive and clinically validated tools to assess treatment adequacy is increasingly critical (21, 22). The ODAS, with its multidimensional structure and adaptable interview format, offers a valuable resource to support evidence-based decision-making, guide individualized buprenorphine dose adjustments, and promote more equitable treatment outcomes in diverse care settings. The present study demonstrates that the ODAS shows acceptable psychometric properties in a non-European Hispanic population, consistent with results from the original Spanish validation and subsequent European studies. Similar to findings in Spanish and German cohorts, we observed good internal consistency and evidence of construct validity, supporting the robustness of the ODAS framework across diverse cultural contexts. However, important differences also emerged. In contrast to European samples, where higher proportions of patients achieved and sustained dose adequacy, our data reflect lower frequencies of dose adequacy, partially explained by features that are common in this population but rarely reported in European cohorts; frequent divided dosing practices. These contextual differences underscore that while the ODAS can reliably capture dose adequacy across populations, interpretation of scores must consider local patterns of treatment delivery and patient behavior.

Clinically, this work highlights the importance of validating measurement tools beyond their original populations. For non-European Hispanic groups, particularly those in high-risk communities with variable treatment adherence, the ODAS offers a structured method for assessing dose adequacy that accounts for the realities of practice. Incorporating the ODAS into care in such settings may support more precise dose adjustments and improved treatment retention. By documenting both consistencies and differences relative to European validation studies, our findings contribute to a more generalizable evidence base for the ODAS and emphasize the need to tailor implementation strategies to cultural and clinical contexts.