20 CFR 416.926a
DI 25225.001 Functional Equivalence – Policy (Section 416.926a(a))
If you have a severe impairment or combination of impairments that does not meet or medically equal any listing, we will decide whether it results in limitations that functionally equal the listings. By "functionally equal the listings," we mean that your impairment(s) must be of listing-level severity; i.e., it must result in "marked" limitations in two domains of functioning or an "extreme" limitation in one domain, as explained in this section. We will assess the functional limitations caused by your impairment(s); i.e., what you cannot do, have difficulty doing, need help doing, or are restricted from doing because of your impairment(s). When we make a finding regarding functional equivalence, we will assess the interactive and cumulative effects of all of the impairments for which we have evidence, including any impairments you have that are not "severe." (See DI 25220.005.) When we assess your functional limitations, we will consider all the relevant factors in DI 25210.001 ff., DI 25215.001 ff., and DI 24501.020 including, but not limited to:
How well you can initiate and sustain activities, how much extra help you need, and the effects of structured or supportive settings (see DI 25210.020);
How you function in school (see DI 25210.030) ; and
The effects of your medications or other treatment (see DI 25210.040).