Generally, Pennsylvania law automatically allows a judgment creditor to increase the judgment amount to account for accruing interest.
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Judgments almost always carry interest. If it is not granted by law, then the judge usually adds it in on the final order of judgment. You're probably also paying attorneys fees and court costs as well.
The legal rate of interest on a judgment in PA is 6% simple interest, not an interest rate on the debt, as a loan or credit card. It sounds like the judgment is a few years old. While it may be worth fighting the interest charges, you probably are better off settling this matter and getting on with your life, because it does not sound like the creditor is going away.