If you had a condition before entering military service and that condition was permanently made worse by your service, VA can grant service connection on an aggravation basis. However, the rating you receive will only compensate for the degree of worsening caused by service, not for the entire disability. VA determines the baseline severity of your condition before service and subtracts that from your current level of disability. The difference is your aggravation rating. For example, if your pre-existing knee condition would currently be rated at 30 percent but was only at a 10 percent level before service, your aggravation rating would reflect the 20 percent increase attributable to service. This baseline deduction can significantly reduce your compensation compared to a full service-connection rating. The presumption of soundness is important in aggravation cases. If your entrance examination did not note the pre-existing condition, VA must prove by clear and unmistakable evidence both that the condition existed before service and that it was not aggravated by service. This is a very high burden of proof. If VA cannot meet this standard, you should be granted full service connection rather than an aggravation rating. Understanding this distinction can make a significant difference in your compensation.
Note: This article references sections of the VA's M21-1 Adjudication Procedures Manual. The VA periodically reorganizes the M21-1 and section numbers may have changed since this article was written. For the most current section references, visit the VA's public M21-1 Web Automated Reference Material System (WARMS).