Question

The patient has an order for cephalexin (Keflex) 350 mg orally. The medication available is cephalexin 250 mg/2 mL. How many mL will the nurse administered

Answer 1

Before we continue with calculation, we will try to estimate our answer. This is really possible only with such simple questions.

We have ; 250 mg in 2ml.

So,we can understand 125 mg in 1 ml (250/2).

Therefore, 375 mg in 3 ml (2ml+1ml).

So,it is clear that 350 mg will be present in within range of 2-3 ml.

Step 1

Write down what we have,

250 mg in 2 ml.

Step 2

Calculate how much one unit of what we have, that is,

250 mg in 2 ml

1mg of keflex = 2÷250 ml

(using one unit rule)

Step 3

We have to find how much ml of keflex is needed to give dosage of 350 mg.Therefore multiply the amount got from step 2 by 350.

350 mg = (2÷250)×350

= 2.8 ml

Answer : we will have to give 2.8 ml of keflex to the client.

This answer correlates with our initial estimation. 2.8 ml is within the range of 2 - 3 ml.

From the above, a formula can be derived;

(amount want ÷ amount have) × volume it's in

Remember care must be taken when using formula. Ensure that numbers are entered and calculated correctly.

Substituting the values in this formula,

Amount want = 350mg

Amount have = 250 mg

Volume it's in = 2 ml

So,

(350÷250)×2 = 2.8

You can apply this method to whatever type of such calculations.

Tip: several ways are there for solving these types of calculations. It is best to learn one way and stick on it.

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