Background
The majority of children with cerebral palsy (CP) experience challenges in functional communication from a young age. A pivotal aspect of functional communication is language comprehension. A variety of classification systems and questionnaires are available to classify and describe functional communication skills in children with CP. A better understanding of the convergent validity of (subsections of) these tools, as well as their relationship with spoken language comprehension, will be valuable in both clinical practice and research.

Aims
To investigate the convergent validity of (subsections of) functional communication tools and the relationship with spoken language comprehension in children with CP.

Methods & Procedures
Cross-sectional data on 138 children were subdivided into three developmental stages based on (Dutch) educational phases: ages 18 months–3;11y (n = 59), 4;0–5;11 years (n = 37) and 6;0–8;11 years (n = 42). The following functional communication tools were used to classify and describe functional communication: Communication Function Classification System (CFCS), subscales of the Caregivers Priorities and Child Health Index of Life with Disabilities—Dutch Version (CPCHILD-DV) and the Focus on Communication Under Six-34 (FOCUS-34) questionnaire. Spoken language comprehension was assessed with the Computer-Based instrument for Low motor Language Testing (C-BiLLT). Correlations between the functional communication tools, and with the C-BiLLT, were calculated using Pearson's and Spearman's correlation coefficients. It was hypothesized a priori that correlations of at least 0.60 suggest good convergent validity.

Outcomes & Results
At all developmental stages, a significant ordered decreasing tendency of communication outcomes was found across CFCS levels; lower CFCS levels were associated with lower scores on the CPCHILD-DV and FOCUS-34, and with a lower level of spoken language comprehension (C-BiLLT). Correlation coefficients of the functional communication tools exceeded 0.60 at all developmental stages. Correlations between C-BiLLT raw scores and the functional communication tools varied between 0.351 and 0.591 at developmental stage 18 months–3;11 years, between 0.781 and 0.897 at developmental stage 4;0–5;11 years, and between 0.635 and 0.659 at developmental stage 6;0–8;11 years.

Conclusions & Implications
The functional communication tools assessed in this study showed convergent validity at all developmental stages. The CFCS, currently most widely used in paediatric rehabilitation, is adequate in the classification of functional communication. However, for more detailed clinical goal setting and evaluation of change in functional communication, the additional use of FOCUS-34 or CPCHILD-DV is recommended.