The debt ratio is simply the ratio of the stock of debt to GDP. So, the debt ratio will rise whenever the increase in debt is greater than the increase in GDP. Even if the primary deficit is zero, the debt ratio will increase if the interest payments on the existing debt are such that they more than offset the fact that the economy is growing. So, a comparison of the real interest rate and growth rate of GDP will be important. Also, the debt ratio will rise if the primary deficit exists (or increases).