University of Pittsburgh Medical Center's net profit dropped 70 percent in the first six months of this fiscal year. The decrease is due largely to a sizable drop in its investment income, a $10 million outlay to a city tuition program and depreciation costs from the system's many improvement projects. Officials representing the system, however, are asking that attention be paid to a measure of cash flow showing the nonprofit earning $260 million before interest expense, depreciation and amortization.