Data from the Patient Registry showed that 1.1 % of boys and 0.3 % of girls had received an autism diagnosis by the time they were eight years old. We found an increase in the use of the diagnosis in all age groups that were included in our study. It is therefore probable that a considerably higher number of children will receive an autism diagnosis before they turn 18.
The findings of our review of patient records suggest that the autism diagnosis is being used in accordance with research diagnostic criteria in Norway. When a doctor or psychologist had determined the diagnosis, the records of 95 % of cases documented that the children met the diagnostic criteria for autism (11). The findings contrast with a similar study of hyperkinetic disorder, where we found that only 49 % of diagnoses were robustly documented in the records (15). We believe that the high standard of documentation of autism diagnoses is thanks to an extensive use of standardised diagnostic instruments, as recommended (10).
Most of the autism assessments included ability testing and direct observation of the child (10). However, only a minority of the children had been assessed for language and adaptive skills. It is important that these functions are assessed in everyone, because the autism diagnosis indicates difficulties with communication and social interaction, but says nothing about impairments in other areas. In DSM-5 and ICD-11, a description of speech, levels of ability and adaptive skills must be included in the diagnosis (6, 7).
Our study has several limitations. A review of patient records is not an independent validation of a diagnosis, as we have not examined the children ourselves. However, the findings from our review of patient records coincide with the validation of autism diagnoses undertaken in the Autism Study, which confirmed the diagnosis in 95 % of cases. The review of patient records covered too few participants in each county for us to analyse possible causes of the variation, such as access to staff and resources, and socio-economic factors such as income, education and the percentage of immigrants in the resident population. Another limitation was the fact that many records were reviewed by a single psychologist. Ideally, all records should have been reviewed by two medical professionals.
The great majority of autism diagnoses were well documented, which indicates that the autism diagnosis is determined in accordance with the diagnostic criteria even in the counties where a higher proportion of children receive the diagnosis. This suggests that in the counties that demonstrate a low rate of prevalence, the health service fails to recognise autism in some children.
It is well known that autism is more common in boys than in girls. The boy-girl ratio is normally around 4 (16) in clinical practice, whereas screening studies, where all children in a certain cohort are assessed for autism, reveal a ratio of approximately 3 (16). We found that the boy-girl ratio is 4.4, and higher in some counties. This may suggest that some girls with autism remain undiagnosed, or that they are diagnosed at a later stage. This would match the findings of the Children in Bergen study, which showed that girls with developmental or mental problems had a lower probability of being seen by the specialist health service than boys with similar levels of symptoms (17).
In the cases where recorded autism diagnoses had not been documented, this was most commonly due to incorrect coding or because the diagnosis had been set tentatively while the assessment was still on-going. Institutions within the specialist health service must abstain from using F84 codes tentatively before a doctor or psychologist has given a conclusive diagnosis of autism.
Our findings do not exclude the possibility that overdiagnosis of autism may occur in Norway. Use of the diagnosis is increasing in all age groups. Diagnoses are often required in order to trigger assistive measures in the education system, which may contribute to a lowering of the threshold for setting the diagnosis. When figures from Stockholm show that as many as 2.5 % of teenagers have received an autism diagnosis (2), it is a timely question whether overdiagnosis occurs. It is important to watch how the diagnostic practice develops in Norway and to be on guard against overdiagnosis as well as underdiagnosis of autism.