Compared with other psychiatric disorders, the risk of misdiagnosis could increase when diagnosing autism spectrum disorder (ASD), although this may be true for all diseases. This increased risk is due to the characteristics of ASD, which continues to the typical development with peculiar traits, and the state of the ASD individual changes with the developmental stages and living environment. In addition, recent active enlightenment in this field may lead psychiatrists to the certain preconception of immediately considering a "developmental disorders". A preconception is a fixed idea or opinion based on limited initial knowledge, which hinders free thought. To surmount preconceptions when diagnosing ASD, this article discusses easy and practical ways for accurately diagnosing ASD, using terminologies related to screening and cognitive psychology.
 In a screening procedure, the positive-predictive value is the rate of accurate diagnoses obtained from the positive results of a screening test. It is expressed using the following numerical formula: [true positive]/([true positive]+[false positive]). This may be comparable to an attitude of raising the diagnostic precision by identifying an ASD characteristic in patients who have been suspected the diagnosis. However, in cognitive psychology, "confirmation bias" is a cognitive bias that anyone can easily fall into. This can occur when someone expects a certain hypothesis and then collects more information that supports the hypothesis, with the application of interpretation and recall biases, resulting in preconceptions. Thus, it is important to cogitate the diagnosis using both proofs and disproofs in order to surmount preconceptions. There is another strategy for raising the positive predictive value and reducing the number of false positives in the screening procedure, using the following numerical formula: (1-[false positive])/([true positive] + [false positive]).
 The concrete clinical considerations are as follows: 1)do not use vague terms of "developmental disorders"; 2)when evaluating present or past ASD characteristics, emphasize concrete findings as much as possible. Because ASD is on a continuum from the quantitative viewpoint using assessment scales, but the qualitative differences are obvious from typical developments. Hence, we should assess whether we can recognize particular ASD symptoms, with consideration of both proofs and disproofs;and 3) when diagnosing psychiatric disorders, we should always broadly consider the possibility of all disorders described under the F0-F9 codes of the ICD-10 classification (along with the possibility of other codes). These considerations will also aid the diagnosis of the comorbidity.
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