Dosage Calculations Practice Test #12: Mixed Review

Answer: A. 0.7 mL

Rationale: Use Desired ÷ Have × Volume: 35 mg ÷ 50 mg × 1 mL = 0.7 mL. 1.43 mL comes from dividing the wrong way (50 ÷ 35). 0.07 mL and 7 mL are decimal-place errors. A quick check: 35 mg is less than the 50 mg in 1 mL, so the answer must be less than 1 mL. Before giving an IM injection, also confirm the volume is appropriate for the client's size and the chosen site; acceptable maximum volumes vary by site, age and facility policy.

Answer: C. 18.2 mL/hr

Rationale: Step 1, find the dose: 12 units/kg/hr × 76 kg = 912 units/hr. Step 2, find the concentration: 25,000 units ÷ 500 mL = 50 units/mL. Step 3, find the rate: 912 units/hr ÷ 50 units/mL = 18.24, rounded to 18.2 mL/hr. 9.1 mL/hr uses 100 units/mL, and 36.5 mL/hr uses 25 units/mL; both come from the wrong concentration. 15.2 mL/hr divides by 60 as if the dose were per minute. Heparin is a high-alert medication: the units/kg/hr starting dose, the weight to use and the titration steps all come from the facility protocol, and many facilities require an independent double check before the infusion starts.

Answer: D. 3.5 units/hr

Rationale: Find the concentration first: 125 units ÷ 250 mL = 0.5 units/mL. Then multiply by the rate: 0.5 units/mL × 7 mL/hr = 3.5 units/hr. 14 units/hr divides the rate by the concentration instead of multiplying. 7 units/hr assumes the bag is 1 unit/mL, which is a common concentration but not this one; always read the label. 0.5 units/hr is the concentration, not the dose. Insulin infusions are titrated to frequent blood glucose checks according to the prescriber's order or facility protocol.

Answer: B. 2.4 mL

Rationale: Step 1, convert the weight: 176 lb ÷ 2.2 = 80 kg. Step 2, find the dose: 1.5 mg/kg × 80 kg = 120 mg. Step 3, find the volume: 120 mg ÷ 50 mg/mL = 2.4 mL. 5.3 mL uses the weight in pounds (176 × 1.5 = 264 mg), which more than doubles the dose. 24 mL is a decimal-place error. 1.6 mL divides the weight in kg by the concentration and skips the dose step. Weight-based doses should use a measured weight in kilograms, not an estimate or a weight in pounds.

Answer: A. 22.5 mL

Rationale: Desired ÷ Have × Volume: 30 mEq ÷ 20 mEq × 15 mL = 1.5 × 15 = 22.5 mL. 10 mL divides the wrong way (20 ÷ 30 × 15). 40 mL puts the numbers in the wrong places (30 ÷ 15 × 20). 15 mL is the volume for 20 mEq, not 30 mEq. mEq works exactly like mg in the formula. Oral potassium liquid is usually diluted in water or juice as directed on the label or drug reference to reduce GI irritation, and the client's potassium level and renal function should be reviewed first.

Answer: D. 50 g

Rationale: A percent solution means grams per 100 mL. D10W contains 10 g of dextrose in every 100 mL. 500 mL ÷ 100 mL = 5, and 5 × 10 g = 50 g. 5 g uses 1 g per 100 mL (a 1% solution). 10 g is the amount in only 100 mL. 100 g is the amount in 1,000 mL. The same rule works for any percent solution: 0.9% sodium chloride has 0.9 g of sodium chloride in each 100 mL.

Answer: B. 50 mL/hr

Rationale: Find the concentration: 20 g ÷ 500 mL = 0.04 g/mL (40 mg/mL). Then rate = dose ÷ concentration: 2 g/hr ÷ 0.04 g/mL = 50 mL/hr. Another way: 2 g/hr ÷ 20 g × 500 mL = 50 mL/hr. 25 mL/hr would deliver only 1 g/hr. 100 mL/hr would deliver 4 g/hr, double the dose. 5 mL/hr is a decimal-place error. IV magnesium sulfate is a high-alert medication: it runs on a pump, the dose comes from the prescriber's order or facility protocol, and the nurse monitors reflexes, respiratory rate and urine output for signs of toxicity as the facility requires.

Answer: C. 6.4 mL

Rationale: Step 1, daily dose: 30 mg/kg/day × 16 kg = 480 mg/day. Step 2, per dose: every 8 hours means 3 doses a day, so 480 mg ÷ 3 = 160 mg per dose. Step 3, volume: 160 mg ÷ 125 mg × 5 mL = 6.4 mL. 19.2 mL is the whole day's amount given at once. 3.9 mL divides the wrong way (125 ÷ 160 × 5). 4.8 mL divides the daily dose by 4, as if the order were every 6 hours. Before giving it, also compare the dose with the safe range in the drug reference, and use an oral syringe to measure the volume.

Answer: D. 42 gtt/min

Rationale: gtt/min = volume (mL) × drop factor (gtt/mL) ÷ time (minutes). 250 × 15 = 3,750. 3,750 ÷ 90 = 41.67, rounded to 42 gtt/min. 28 gtt/min uses a 10 gtt/mL drop factor. 167 is the rate in mL/hr (250 mL ÷ 1.5 hr), not drops per minute. 63 gtt/min uses 60 minutes instead of 90. Always use the drop factor printed on the tubing package; drop factors differ between tubing sets. Gravity infusions need to be checked often because the rate can change when the client moves.

Answer: A. Give 5.0 mg orally daily

Rationale: A trailing zero after a decimal point (5.0 mg) is on The Joint Commission's Do Not Use list. If the decimal point is missed, 5.0 mg can be read as 50 mg, a tenfold overdose. 0.5 mg is written correctly: a leading zero before a decimal point is required, because .5 mg can be misread as 5 mg. 5 mg is a whole number with no decimal. 500 mcg is an acceptable way to write the same amount as 0.5 mg; many facilities prefer whole numbers in mcg for small doses. Follow your facility's list of error-prone abbreviations and dose expressions.