There is a common misconception about this. The simple answer is "no" -- a test of intllectual functioning is not required.
Here's the reasoning, broken down into its components:
42 CFR 483.136(b) – "Data" -- lists the data elements that must be collected to determine whether someone has an intellectual disability. Note that an IQ test is not among the required elements.
42 CFR 483.136(c) – "Data Interpretation" – indicates that "the State must ensure that a licensed psychologist identifies the intellectual functioning measurements of individuals with ID or a related condition." A couple of very important things about this clause:
- The fact that it is labeled interpretation means only that tests of intellectual functioning, when they are administered, must be interpreted by a licensed psychologist. It does not say that such a test is required.
- "Licensed psychologist" does not necessarily mean someone with a PhD. Instead, it means whoever the state chooses to license, according (e.g.) to its practice acts.
The CFR incorporates by reference the 1983 manual of what was then called the American Association on Mental Deficiency (AAMD). In its definition of "significantly subaverage IQ," the manual says the following:
Significantly subaverage IQ is defined as IQ of 70 or below on standardized measures of intelligence. This upper limit is intended as a guideline [emphasis mine]; it could be extended upward through IQ 75 or more, depending on the reliability of the intelligence test used. This particularly applies in schools and similar settings if behavior is impaired and clinically determined to be due to deficits in reasoning and judgment.
A copy of the relevant chapter can found here.
In other words, the manual gives states a great deal of flexibility; an IQ of 70 is just an anchor, or a point of reference. It's not a firm cutoff. And information about intellectual functioning came come from school and other settings.
The bottom line is that you do not need an IQ test if is impractical to conduct one; a good psychosocial history will do just fine, especially if there's a family historian around. The IQ test, if available, need not be current. And if you conduct an IQ test, you don't need a PhD psychologist to interpret it – you just someone licensed by the state.