It is the most sensitive diagnostic test available and is used first line in all cases, but can miss balanced translocations and low-level mosaicism. Specific gene testing is available for certain disorders such as Rett syndrome, although these are expensive tests which arent widely available
Selective metabolic investigations may be useful in the absence of other identifiable causes, and the specific tests done will depend on the presentation. Inborn errors of metabolism causing metabolic disorders are rare and there are limited treatment options even if they are successfully diagnosed. Targeted MRI brain can be considered second line in selected patients, and is more likely to contribute to a diagnosis if the child has abnormal physical signs such as microcephaly, macrocephaly, a change in head circumference, focal neurological signs, or epilepsy.Neonatal screening is used in the UK (Guthrie test) and can diagnose certain inborn errors of metabolism before they cause significant developmental problems, with the aim to manage them so that no permanent damage occurs. Medium-chain acyl-CoA dehydrogenase deficiency
Homocysteinuria
Congenital hypothyroidism
Isovaleric acidaemia
Glutaricaciduria type 1
Maple syrup urine diseaseCanada, the US, and the Netherlands offer more extensive newborn screening, encompassing some other amino acid, organic, and urea cycle disorders

Management
The specific management of children with global developmental delay will depend on their individual needs and underlying diagnosis. Early intervention is essential to support the child to reach their full potential.
Even though there are many known causes of delay, some children will never receive a diagnosis. Investigation
Developmental monitoring is performed during wellness visits to check a childs development. Health authorities encourage parents to monitor their childs development, the CDCs program "Learn the Signs. Act Early" provides materials for a childs development is assessed based on expected milestones for actions like how they play, learn, speak, act and move. Missed milestones may be cause for concern, so the doctor or another specialist may call for a more thorough test or exam to take a closer look, this is usually done by going through Developmental Screening. Developmental Screening is a more involved process. The evaluating professional will ask a parent to complete a research-based questionnaire that asks about a childs development, including language, movement, thinking, behavior, and emotions. Developmental Screening is recommended by the American Academy of Pediatrics (AAP) to all children at 9, 18, and 30 months. The AAP also recommends that all children be screened specifically for autism spectrum disorder (ASD) during regular well-child visits at 18 and 24 months. If a Developmental Screening indicates a delay, the child should then be assessed with a Developmental Evaluation. Developmental Evaluations are performed by a Developmental pediatrician, child psychologist, or other trained provider with the purpose of Identifying and diagnosing developmental delays and conditions. Chromosome microarray and karyotyping to look for trisomy, microdeletions, and duplications.