🧮 剂量计算 · 第2课，共5课 · 6 min
所需剂量除以现有剂量，再乘以数量
Desired Over Have, Times Quantity
💊 医嘱是500 mg。瓶身标注为250 mg每5 mL。一个公式就能解答这类每一个问题，而考试可能会给你出好几道这样的题。
💊 The order is 500 mg. The bottle says 250 mg per 5 mL. One formula answers every question of this type, and the exam can give you several of them.
💡 核心要点
所需剂量 ÷ 现有剂量 × 数量，并保持单位一致。如果答案看起来不合理，那就是换算错了。
Desired ÷ Have × Quantity, with matching units. If the answer looks absurd, the conversion is wrong.
🧠 为什么重要
公式：所需剂量 ÷ 现有剂量（手头有的剂量）× 数量（其所在的容量或剂型）= 应给予的量。医嘱500 mg，现有250 mg每5 mL：500 ÷ 250 × 5 = 10 mL。医嘱0.25 mg，现有0.125 mg片剂：0.25 ÷ 0.125 × 1 = 2片。所需剂量和现有剂量的单位必须一致，所以先换算：医嘱1 g，现有500 mg片剂，将1 g换算为1,000 mg，然后1,000 ÷ 500 = 2片。量纲分析法通过排列单位使其相互抵消，也能得出相同的答案。对结果进行合理性检查：15片或0.02 mL的剂量几乎总是意味着换算错误。对于注射剂，选择能准确量取容量的注射器：小剂量使用1 mL注射器，并四舍五入到注射器的刻度。只掰开带有刻痕可供掰开的片剂，并且除非产品标签允许，否则绝不掰开或碾碎肠溶片或缓释片。
The formula: DESIRED dose ÷ HAVE (the dose on hand) × QUANTITY (the volume or form it comes in) = amount to give. Order 500 mg, have 250 mg per 5 mL: 500 ÷ 250 × 5 = 10 mL. Order 0.25 mg, have 0.125 mg tablets: 0.25 ÷ 0.125 × 1 = 2 tablets. The units of desired and have must MATCH, so convert first: order 1 g, have 500 mg tablets, convert 1 g to 1,000 mg, then 1,000 ÷ 500 = 2 tablets. Dimensional analysis gives the same answer by lining up units so they cancel. Sanity-check the result: a dose of 15 tablets or 0.02 mL almost always means a conversion error. For injectables, choose the syringe that measures the volume accurately: a 1 mL syringe for small doses, and round to the syringe's gradations. Split only tablets that are scored for splitting, and never split or crush enteric-coated or extended-release tablets unless the product labeling allows it.
🏥 真实场景
🧴 医嘱：amoxicillin 400 mg PO。现有：250 mg每5 mL的混悬液。400 ÷ 250 = 1.6；1.6 × 5 = 8 mL。护士用口服注射器量取8 mL。一位忘了乘以数量的同学给出了1.6 mL，是剂量的五分之一。公式有三个部分，因为三个部分都很重要。
🧴 Order: amoxicillin 400 mg PO. Available: 250 mg per 5 mL suspension. 400 ÷ 250 = 1.6; 1.6 × 5 = 8 mL. The nurse measures 8 mL in an oral syringe. A classmate who forgot to multiply by the quantity gives 1.6 mL, one fifth of the dose. The formula has three parts because all three matter.
📌 要点总结
- 所需剂量除以现有剂量，再乘以手头的数量Desired over have, times the quantity on hand
- 相除之前先换算，使单位一致Convert so the units match before dividing
- 不合理的答案提示换算错误Absurd answers signal a conversion mistake
📖 本课术语
所需量除以现有量 (Desired over have): 剂量计算公式：所需剂量除以现有剂量，再乘以其所含的数量。
✅ 自我测试
医嘱开具某种抗生素750 mg，现有的混悬液是500 mg每5 mL，护士应给予多少mL？The order is for 750 mg of an antibiotic. The suspension available is 500 mg per 5 mL. How many mL should the nurse give?
- 5 mL
- 10 mL
- 7.5 mL
- 15 mL
答案: C · 7.5 mL
750 ÷ 500 = 1.5；1.5 × 5 mL = 7.5 mL。5 mL只是标签上的容量，只能提供500 mg，是医嘱剂量的三分之二。
750 ÷ 500 = 1.5; 1.5 × 5 mL = 7.5 mL. 5 mL is simply the volume on the label and would deliver only 500 mg, two-thirds of the ordered dose.
医嘱开具某种药物0.5 g，现有片剂为250 mg，护士应给予多少片？The order is for 0.5 g of a medication. Tablets are available as 250 mg. How many tablets should the nurse give?
- 2片2 tablets
- 1片1 tablet
- 5片5 tablets
- 0.5片0.5 tablet
答案: A · 2片
0.5 g = 500 mg；500 ÷ 250 = 2片。0.5片是将除法颠倒得出的，即250 ÷ 500而不是500 ÷ 250。
0.5 g = 500 mg; 500 ÷ 250 = 2 tablets. 0.5 tablet comes from inverting the division, 250 ÷ 500 instead of 500 ÷ 250.
医嘱要求给予某种注射药物60 mg，药瓶含80 mg每2 mL，护士应抽取多少容量？An order calls for 60 mg of an injectable medication. The vial contains 80 mg per 2 mL. What volume should the nurse draw up?
- 0.75 mL
- 1.5 mL
- 2.5 mL
- 3 mL
答案: B · 1.5 mL
60 ÷ 80 = 0.75；0.75 × 2 mL = 1.5 mL。0.75 mL是停在60 ÷ 80这一步、忘了乘以2 mL（80 mg所在的容量）得出的。
60 ÷ 80 = 0.75; 0.75 × 2 mL = 1.5 mL. 0.75 mL comes from stopping at 60 ÷ 80 and forgetting to multiply by the 2 mL that holds the 80 mg.
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