Drawdown is the average reservoir pressure in the drainage volume minus the flowing bottomhole pressure. For a single-phase oil above the bubble point the index is close to constant, in stock-tank barrels per day per psi. A fall that depletion alone would not explain points to damage, a rising skin, or a loss of effective permeability.
Below the bubble point, and in a gas well, the relationship between rate and flowing pressure curves, and a single index is only a local slope. The inflow performance relationship is the curve; the productivity index is its slope where the curve is straight. Comparing indexes between wells is only fair when the wells have similar fluid, similar drawdown, and a similar definition of average pressure.