Although the catch up is not subject to the testing, it can impact it. This will all depend on how the Treasury decides in its guidance as to how to signify catch up contributions.
For example, let's say that an HCE defers $10,000. Knowing that their plan regularly fails an ADP test, an election is made (assuming Treasury says this method is an allowable one) that any amount that may not be recognized as a elective deferral be automatically treated as a catch up contribution. In the test, let's say that this HCE is limited to $8,500, and gets a $500 catch up.
That seems all clean, except: There is an effect on the under-50 HCEs by doing this. There are ways to construct this example further that hurt the under-50 people. Let's say that because the test failed, some others had their deferrals limited, too. However, they are not allowed catch ups, so their deferrals are returned (or recharacterized as after-tax).
If, on the other hand, this HCE had elected an $8,000 deferral and a $1,000 catch up from the start, it may be that the under-50 people no longer need a return of their deferrals and the test passes. Of course, our HCE now has not hit the limit in the test, so they are not eligible for a deferral. Naturally, a procedure may be in place (assuming Treasury allows this), to shift some of the $1,000 catch up to a deferral because our HCE is not at the limit. In this case, it may only take $100 or $200 to "max out" the ADP test without affecting other HCEs.
So, depending on the procedures in place to determine which money is elective deferral and which is catch up, there can be adverse consequences in the ADP test. These are the issues that Treasury is struggling with in coming up with guidance on how to establish procedures for signifiying what is deferral and what is catch up.
Another example: Assume an HCE designates maximum deferral plus some catch up from the beginning of the year and it is withheld ratably during the year. The HCE terminates late in the year. Now, all of the money may need to kick down to deferral because the person is not eligible for the catch up (did not hit any limit). This could affect the ADP just like the first example.
For a general discussion of the law (no examples like this, though), see:
http://www.milliman.com/empl_ben/publicati...eginfobulletins