Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by early deficits in social skills, communication, and repetitive behaviors [1]. These symptoms can greatly affect daily functioning, affecting both children and their caregivers. Early diagnosis and intervention are crucial to improve long-term outcomes [2].

Although research indicates that ASD can be diagnosed as early as 14 months of age [3], the diagnosis often occurs around the age of 4 years in clinical practice worldwide [4,5]. Therefore, clinicians should utilize time-efficient ASD screening instruments to identify at-risk children and initiate early interventions before confirming the diagnosis.

Research has focused on the development of new instruments and strategies to improve early ASD screening. One approach is to develop ASD screening instruments that consider each country’s cultural context as cultural factors can influence the assessment of ASD symptoms [6,7]. Cultural factors can alter the assessment criteria for ASD symptoms [8]. Efforts have emerged to create culturally adapted tools, especially in Asia, to complement the many ASD instruments developed in the U.S. For example, South Korea developed the Behavior Development Screening for Toddlers-Play/Interview (BeDevel-P/I) for young children [9], Hong Kong introduced the Classroom Observation Scale for preschoolers [10], and India developed the Chandigarh Autism Screening Instrument [11]. Although these culturally tailored tools are beneficial, their widespread adoption in clinical practice is time-consuming. The second strategy is to translate and adapt existing instruments, revalidate them for local populations, and adjust the cutoff points. This method saves time and allows for cross-population comparisons. However, adapting instruments can lead to qualitative differences in assessment owing to variations in the original version, sampling, and psychometric properties [7]. Although efforts have been made to identify culturally appropriate cut-off points, few studies have thoroughly validated these adaptations, especially in East Asia.

Even with standardized screening instruments, their validity often varies when applied in clinical settings, which is also true in South Korea. The effectiveness of the instruments can vary depending on the sampling method [7]. In particular, when there is a qualitative difference between the sample in which the instrument’s validity is confirmed and the sample in which the instrument is actually applied, the comprehensibility of the instrument tends to decrease [12]. In South Korea, although the Social Communication Questionnaire (SCQ) and the Social Responsiveness Scale (SRS) have been validated and are commonly used in clinical settings for ASD screening, the use of these instruments alone is limited in terms of accuracy.

This study aimed to explore ways to enhance the diagnostic yield using the Korean version of the SCQ (K-SCQ) and Korean version of SRS second edition (K-SRS-2), which are parent-report screening instruments for ASD that have been validated for reliability and are commonly used in clinical practice in South Korea. Given the potential benefits of such combinations, we verified the screening accuracy of combining these two instruments in various ways. Additionally, as a strategy to further enhance screening accuracy, we divided the children into age groups and determined the optimal cutoff for each age group.

This study is the first to attempt to combine two level 1 screening instruments for the early detection of ASD and demonstrated that the combination of two primary screening instruments can help reduce delays in ASD screening in community settings.

In children aged ≥30 months, application of the “either K-SCQ or K-SRS-2” resulted in a sensitivity, specificity, and PPV exceeding 80%, demonstrating high validity. The NPV was not as high (64.5%-81.2%) in this age range, which likely reflects the higher proportion of children with non-ASD children without ASD. Thus, for children aged ≥30 months, it appears reasonable to begin early intervention if they are screened as being at risk for ASD using either of the two screening instruments. Conversely, in children under 30 months of age, combining primary screening instruments resulted in high false positives, limiting their use for reliable ASD determination. For the adjusted cutoffs, except for the 10-17 months age range, both screening instruments showed lower values than the originally recommended cutoffs. The age range with the most significant difference from the original cutoff for the K-SCQ was 42-53 months (cutoff, 7.5), and for the K-SRS-2, it was 54-60 months (cutoff, 52.5).

The findings of this study align with those of previous research, highlighting the need to be cautious of overdiagnosis in younger children and suggesting that a more stable diagnosis of ASD can typically be made after 24 months [22,23]. These findings may be partly explained by the inclusion of younger children in the study sample, some of whom were below the recommended age range for the SCQ and SRS when they were originally developed. The SCQ is recommended for children aged 4 years and older, while the SRS-2 is recommended for children aged ≥2.5 years. Therefore, using only primary screening instruments for children under 30 months of age may be inaccurate and lead to high false positives. Previous studies have also suggested that applying the SCQ to children under the age of four may lead to an increased rate of false positives [24]. Incorporating additional level 2 assessments could be a viable solution to improve screening accuracy and reduce false positives in this age group. Caution is necessary when applying screening instruments to an age range broader than originally intended.

Among the children in this study, the validity of the K-SRS-2 was lower than that of the K-SCQ. Additionally, when examining agreement with the Korean-developed level 2 screening instruments BeDevel-P/I, the K-SCQ consistently showed higher agreement than the K-SRS-2. This discrepancy may be attributed to qualitative differences in how the SCQ and SRS-2 assess ASD or to the varying impacts of sociocultural factors on each instrument in the Korean context. Sociocultural factors, emphasized in previous studies as having a significant influence on ASD diagnosis, could also play a role. These factors not only affect the adaptation process of the instruments but also influence how parents perceive and report their children’s autistic traits through screening items [25,26]. Therefore, when applying screening instruments developed in other countries to a domestic context, it is crucial to consider how well these instruments align with the cultural context of the target country. Additionally, considering that previous studies have shown variability in cutoff scores, the fact that this study also found differences in cutoff scores compared to established values, as well as variations across different age groups, suggests that the interpretation of screening instrument items may be influenced by the cultural context in Korea [7].

This study has several strengths. First, it included a large sample size, and ASD diagnosis for participants was conducted by child psychiatrists and other specialists using standardized diagnostic instruments. Additionally, the study utilized two screening instruments commonly used in the Korean healthcare setting that were easily completed by parents through questionnaires. By examining methods to enhance the diagnostic accuracy of ASD and verifying the validity of these tools, this study provides valuable insights. Aligning the screening process with the existing schedule of Korean early childhood health checkups for children aged ≥30 months is expected to significantly aid in the early diagnosis of ASD.

However, this study has some limitations. A method to improve the screening accuracy for children ≤30 months without increasing the risk of misdiagnosis has not yet been proposed. In particular, the sample size for the 10-17-month age group was insufficient, limiting the statistical power and generalizability of the findings within this age range. Moreover, given the substantially higher proportion of participants aged 30-60 months compared to those under 30 months, and the increased stability of their diagnostic processes and outcomes, caution is warranted when interpreting the diagnostic accuracy findings for the younger age group. Another limitation is the insufficient elucidation of the qualitative differences between the two screening instruments. Previous research has shown that parent-reported screening tools can be effectively developed for children aged 12-36 months with significant validity, indicating a potential area for further research to address the limitations of this study [27]. The Early Screening for Autism and Communication Disorders demonstrated high validity, even in children younger than 24 months, by presenting different cutoff values for different age groups and specific items. Because parent-reported screening instruments are convenient and accessible, further research is needed to explore methods to enhance screening accuracy using primary screening instruments in children under 24 months of age. Overall, while this study demonstrates a strong potential for improving early ASD diagnosis through commonly used screening tools in the Korean clinical context, further investigation is needed to refine screening methods for younger children and explore the qualitative aspects of the tools used.

In summary, for children under 30 months of age, using only the primary screening instrument may increase the likelihood of false positives; therefore, employing a secondary screening instrument in conjunction may reduce false positives and improve accuracy. For children aged 30-60 months, both K-SCQ and K-SRS-2 demonstrated good validity. Combining these two qualitatively different tests and considering a child as having ASD if they screened positive in either test resulted in higher sensitivity without significantly increasing false negatives. To improve the diagnostic accuracy of ASD, it is recommended that different cutoff values be applied according to age group. Future research should include further analyses of the K-SCQ and K-SRS-2 to enhance diagnostic accuracy across a wider age range. Specifically, additional analyses should be conducted on other factors influencing ASD screening (such as sex, age, and developmental level) and item-level analysis.