目的：本文針對醫院體系之延後付款制度，發展出適合於二階信用交易下三種採購批量模式，以協助醫院資材室與分醫院庫房擬定經濟採購與補貨批量決策。由於傳統經濟採購批量（Economic order quantity, EOQ）未考慮延後付款因素，若醫院直接使用EOQ模式決定其採購批量，將易導致訂購決策偏誤，而使存貨成本增加。方法：二階信用交易是指醫院同時接受藥品供應商的延後付款期間（M）與建保局的延後收款期間（N）。本論文利用成本函數衡量二階信用交易制度下資材室的採購、儲存、資金積壓成本，以及分醫院補貨、儲存、資金積壓與機會收益，分別導出三種不同採購週期（T）狀況之批量模式（即模式一：T ＜（M-N）、模式二：（M-N）≤ T ＜ M 與模式三：T ≥ M）。再應用偏導數與迭代演算法分別求解出三種批量模式的最佳採購週期、補貨次數與成本極小值。結果：以大千綜合醫院個案為範例，經過模式求解結果發現，傳統EOQ模式求得採購週期為最短，模式三求得採購週期為最長。模式一求得最低的存貨成本，並比傳統EOQ成本低達9%。本文建議醫界可使用模式一規劃兩階層庫房採購批量決策。結論：醫院體系在延後付款制度下，應向供應商爭取較長的延後付款期間並同時節約採購成本，此將可大幅降低存貨成本。另外，醫院應尋找較高的資金投資獲，以賺取可觀的延後付款機會收益。

Objective: Credit transactions are widely used in the healthcare industry; however, the traditional economic order quantity (EOQ) model does not take delayed payments into account. If a purchasing manager adopts this EOQ formula to make purchasing decisions, then it will increase inventory cost. This paper presents three EOQ models utilizing two levels of trade credit to determine the optimal ordering and replenishment decisions for hospital purchasing departments.Method: Two levels of trade credit mean that the supplier provides the hospital with a permissible delay period (M) for payment and the hospital also accepts a permissible delay period (N, M ＞ N) from the national health insurance program. This study utilized permissible delays in payments to evaluate ordering costs, storage costs, capital opportunity costs, and interest earned for the hospital system. According to the length of ordering interval (T), three EOQ models: Model 1: T ＜ (M-N), Model 2: (M-N) ≤ T ＜ M, and Model 3: T ≥ M) were formulated. Partial derivatives and iterative algorithms were used to find the optimal solutions for these EOQ models. These included the ordering interval, the number of replenishments, and the minimal inventory cost.Results: Using data from the Da-Chien hospital, the results showed that the traditional EOQ model had the shortest ordering interval, whereas model 3 provided the longest ordering interval. Model 1 resulted in the lowest cost which was about 9% less than the cost of the traditional EOQ model. We propose that model 1 is appropriate for a two-echelon hospital system to make purchasing and replenishment decisions.Conclusions: In a delayed payment situation, the hospital purchasing manager should negotiate with suppliers to get a longer permissible delay period and should reduce ordering cost in order to receive a significant saving in inventory cost. Meanwhile, the financial manager should take advantage of this opportunity to seek a higher return rate on capital investment.