Dosage Calculations Practice Test #8: Reconstitution

Answer: C. 1.4 mL

Rationale: Use the concentration the label gives after mixing, not the diluent volume. 500 mg ÷ 350 mg/mL = 1.43, rounded to 1.4 mL. 1.1 mL is the error of dividing 1,000 mg by 2.1 mL (476 mg/mL): the powder itself takes up space, so the final volume is larger than the diluent added and the true concentration is lower. 0.7 mL comes from flipping the fraction (350 ÷ 500). Labels differ by manufacturer and by route (IM labels may call for sterile water or 1% lidocaine), so always use the directions on the vial in your hand.

Answer: A. 3 mL

Rationale: The nurse added 8.2 mL, so the concentration is 500,000 units/mL. 1,500,000 units ÷ 500,000 units/mL = 3 mL. 1.5 mL would be correct only if the vial had been mixed to 1,000,000 units/mL, and 6 mL only at 250,000 units/mL. This is why the chosen concentration must be written on the vial label: the same vial can hold three different strengths. Use the table on the product you have, because diluent volumes vary between manufacturers.

Answer: D. 3 mL

Rationale: The powder displaces 0.4 mL, so the final volume is 10 mL, not 9.6 mL. Concentration: 2,000 mg ÷ 10 mL = 200 mg/mL. Volume: 600 mg ÷ 200 mg/mL = 3 mL. 2.9 mL is the displacement error: 2,000 mg ÷ 9.6 mL = 208.3 mg/mL, then 600 ÷ 208.3 = 2.88. Always base the concentration on the final volume or the concentration the label states.

Answer: B. The date and time it was mixed, the final concentration, and the nurse's initials

Rationale: Once reconstituted, a drug becomes unstable over time, and the vial can be mixed to more than one strength. The label needs the date and time of mixing, the concentration made, and the initials of the nurse who mixed it, plus the beyond-use date or time and storage instructions (for example, refrigerate) from the package insert. The manufacturer's printed expiration date applies only to the dry powder. How long a reconstituted drug is stable varies by product, so follow the insert and your facility policy.

Answer: D. 5 mL

Rationale: Daily dose: 25 mg/kg/day × 20 kg = 500 mg/day. Every 12 hours = 2 doses, so 500 ÷ 2 = 250 mg per dose. Volume: 250 mg ÷ 250 mg/5 mL = 5 mL. 10 mL is the full daily amount given as one dose. Shake reconstituted suspensions well before measuring, use an oral syringe, and check the storage and discard date on the bottle. Pediatric doses depend on the indication and the prescriber's order, so always verify the dose falls within the safe range for that child.

Answer: C. 15 mL

Rationale: 750 mg ÷ 50 mg/mL = 15 mL. 20 mL would add the whole 1 g vial (an overdose of 250 mg). 1.5 mL is a decimal error. After it is added to the piggyback, vancomycin must be further diluted and infused slowly; the infusion time and final volume come from the prescription, the package insert and facility protocol, because infusing too fast can cause flushing, rash and hypotension (vancomycin infusion reaction).

Answer: A. 6 mL

Rationale: Convert first: 1.5 g = 1,500 mg. Volume: 1,500 mg ÷ 250 mg/mL = 6 mL. 5.1 mL is the error of using the diluent volume (1.5 g is 3/4 of the vial, and 3/4 of 6.8 mL = 5.1 mL); the powder adds volume, so the vial holds about 8 mL after mixing. 0.6 mL is a decimal error from mixing up g and mg.

Answer: B. 6 doses

Rationale: Each dose: 150 mg ÷ 200 mg/mL = 0.75 mL. Doses in the vial: 5 mL ÷ 0.75 mL = 6.67, or 1,000 mg ÷ 150 mg = 6.67. Only 6 full doses are available; you cannot round up to 7, because the seventh dose would be short. In practice, a reconstituted vial may expire before all doses are used, so check the beyond-use time on the label and discard what remains after it.

Answer: A. 4.8 mL

Rationale: Dose: 7.5 mg/kg × 64 kg = 480 mg. Volume: 480 mg ÷ 100 mg/mL = 4.8 mL. The answer happens to match the diluent volume, but the reasoning is different: the vial holds 5 mL after mixing (500 mg at 100 mg/mL), and the client needs 480 of those 500 mg. 5 mL would give the full 500 mg. 0.48 mL is a decimal error. Weight-based doses depend on the drug and the order, so check the result against the safe range in the drug reference.

Answer: B. Obtain the diluent specified on the label, or ask the pharmacist before mixing

Rationale: Use only the diluent and volume the label specifies. Some drugs are incompatible with sodium chloride or with the preservative in bacteriostatic diluents (benzyl alcohol, which must not be used in neonates), and the wrong diluent can make the drug precipitate or change its strength. Changing the volume changes the concentration, so every later calculation would be wrong. If the right diluent is not available, get it from pharmacy or ask the pharmacist before mixing.