drug dosage calculations

Question

Calculate the amount you'll prepare for each dose.

1. Order: cyanocobalamin 0.5mg IM once/week.
Give: ____mL
What's on the medicine label: 10,000mcg/10mL

 

2. tinzaparin sodium 50 units/kg subcut 2 h before hip surgery. Patient weighs 100kg.
Give: ____mL
Label: 20,000 anti Xa IU/2 mL

 

3. hydromorphone hydrochloride 2mg IM stat

Give: _____mL

Label: 50mg/5mL

 

For all of these, the answers are 0.5mL according to the textbook but I get different answers...

Answer 1

1. 0.5mg=500mcg. 10,000mcg/10ml=1000mcg/ml, so 500mcg in 0.5ml. The amount to be administered is 0.5ml.

2. 50 units/kg requires 50×100=5000 units for a 100kg patient.

20,000 anti Xa IU/2ml=10,000 units/ml and we need half this number of units, so we need 0.5ml.

3. 50mg/5ml=10mg/ml. We need 2mg which is one fifth ml=0.2ml. 0.5ml contains 5mg of the drug.