Short forms of IQ (S-IQ) assessments are time efficient and highly predictive of the full IQ (F-IQ) in healthy individuals. To investigate the validity of S-IQs for patients with neurodevelopmental impairments, this study tested a well-established S-IQ version in patients with congenital heart disease (CHD).The Wechsler Intelligence Scale for Children, Fourth Edition was applied in 107 children with complex CHD aged 9-11 years. F-IQ and a well-established S-IQ version were calculated for each patient. The agreement between S-IQ and F-IQ was investigated across the whole spectrum of IQ scores. Finally, we tested a method to adjust IQs to resolve potential bias and validated this method in an independent sample of 55 CHD patients.S-IQ and F-IQ correlated strongly. Nevertheless, the size of the bias correlated with the true IQ, indicating larger error at the tails of the distribution. Estimating a corrected IQ by adjusting the S-IQ with correction parameters substantially improved agreement.We here report that substantial bias may underestimate low IQ scores and overestimate high ones. This bias should be considered when at-risk populations are assessed with S-IQs. Importantly, the bias can be minimized by using a correction formula.