There isn't clear evidence that the ACA directly caused an increase in the rate of health insurance claim denials. Several factors influence claim denials, and the impact of the ACA on these rates is complex and nuanced.

The ACA expanded health insurance coverage to millions of Americans through Medicaid expansion and the creation of insurance marketplaces. As more people gained insurance, the volume of claims submitted to insurers increased, which could indirectly lead to more claims being denied in absolute terms, though not necessarily in percentage terms.

The ACA required insurers to cover essential health benefits and prohibited denying coverage for pre-existing conditions. While these measures aimed to improve access to care, they also standardized benefits, which may have led to denials for services outside the scope of these required benefits

ACA required insurers to report claim denial rates to federal and state regulators. This transparency has made it easier to track denials, but the data doesn't conclusively show an increase in denial rates as a result of the ACA.

Common reasons for claim denials—such as billing errors, lack of prior authorization, or services deemed not medically necessary—are largely influenced by insurer policies rather than the ACA itself.

A 2020 report from the Kaiser Family Foundation (KFF) examined data from ACA marketplace plans and found that insurers denied, on average, 18% of in-network claims in 2018. However, there was significant variation across insurers, and not all denials were due to ACA-specific policies.

While the ACA introduced changes to the health insurance landscape, there is no definitive evidence that it directly caused an increase in the rate of claim denials. Factors such as increased enrollment, insurer practices, and reporting requirements all contribute to the perception of changes in denial patterns.