Why not? In 2016, the U.S. Preventive Services Task Force concluded that "current evidence is insufficient to assess the balance of benefits and harms of of screening for autism spectrum disorder (ASD) in young children for whom no concerns of ASD have been raised by their parents or a clinician." The Task Force observed that most ASD treatment studies included children who were considerably older than those identified through screening, and that no controlled studies have looked at the comparative clinical outcomes of screening-identified children with ASD, which is what a guideline writer would definitely want to know before recommending universal screening, even if the AAP didn't think so.
Dr. Doug Campos-Outcalt, a longtime colleague who has served as the American Academy of Family Physicians' liaison to the USPSTF, wrote in American Family Physician that four critical questions needed to be answered before screening for ASD would be "ready for prime time":
1. What are the sensitivity and false-positive rate of the best screening test for ASDs available in an average clinical setting?
2. How much earlier can screening tests detect ASDs compared with an astute clinician who asks a few key questions about, and acts on, parental concerns regarding a child's communication and interactions?
3. What are the potential harms of testing?
4. Does earlier detection by screening result in meaningful and long-lasting improvements compared with detection through routine care?
Although the answers to the second and fourth questions are arguably the most important, until last week there was little evidence to answer the first and third, either. If the recommended screening test, the Modified Checklist for Autism in Toddlers, Revised, with Follow-Up (M-CHAT-R/F), can't reliably detect most children who will eventually develop symptoms of ASD in later life, or there are so many false positives that the harms of parent anxiety and unnecessary diagnostic evaluations would outweigh the benefits, then universal screening is unlikely to work. Unfortunately, the first large study (n=26,000) of near-universal screening for ASD in 31 primary care clinics affiliated with Children's Hospital of Philadelphia just provided disappointing results on both of these fronts. Using an older version of the M-CHAT, the sensitivity of screening was only 38.8%, and only 14.6% of children who screened positive ultimately received an ASD diagnosis, with even lower positive predictive value in children residing in lower-income households.
The authors pointed out that nearly 60% of children with initial positive screens did not return for a follow-up interview that might have reduced false positives and improved predictive value, and that children with positive screens who were diagnosed with ASD were more likely to receive interventions at a younger age, potentially improving outcomes. But the former simply shows how a two-stage screening test performs in real life, rather than in a controlled research setting. As for the latter, outside of anecdotes from screening advocates, we still have no conclusive evidence that long-term outcomes turn out better in these children. The bottom line? For universal screening for ASD in toddlers, the jury is still out.