In a 2024 meta-analysis of MRI/fMRI machine-learning studies for ADHD, the average pooled sensitivity was 74% (the ability to correctly identify people who actually have ADHD) and the average pooled specificity was 75% (the ability to correctly identify people who do not have ADHD), that also means that there is a 26% false-negative rate (miss/Type II error) and a 25% false-positive rate (accidental diagnosis/Type I error) [https://doi.org/10.1016/j.jad.2024.03.111].

Though there is no set standards but under 80% is pretty low for a clinically useful test but can still be used in some conditions such as to supplement diagnosis. To give some numbers the rapid antigen covid tests had 72% sensitivity and 98.9% specificity. (note the lower sensitivity was acceptable at the time but the CDC guideline was for symptomatic people to retest 48 hours. Viral load and other staging factors could all confound.) [https://doi.org/10.1371/journal.pmed.1004011]