Numeracy: Injections, Infusions & Fluid Balance

Enoxaparin 40 mg is prescribed. The pre-filled syringe contains 80 mg in 2 mL. What volume should be given?

• A1 mL✓
• B0.5 mL
• C2 mL
• D0.8 mL

Answer: Volume = (40 ÷ 80) × 2 mL = 0.5 × 2 = 1 mL. Working in the syringe's own concentration (40 mg per mL) gives the same answer.

1,000 mL of sodium chloride 0.9% is to run over 8 hours through an infusion pump. What rate should the pump be set to?

• A150 mL/hour
• B125 mL/hour✓
• C80 mL/hour
• D100 mL/hour

Answer: Rate in mL per hour = total volume ÷ hours = 1,000 ÷ 8 = 125 mL/hour.

500 mL of fluid is to be given over 4 hours through a standard giving set (20 drops per mL). What is the drip rate, to the nearest whole drop per minute?

• A21 drops/min
• B125 drops/min
• C42 drops/min✓
• D50 drops/min

Answer: Drops per minute = (volume × drop factor) ÷ time in minutes = (500 × 20) ÷ 240 = 10,000 ÷ 240 = 41.7, rounded to 42. 125 is the rate in mL per hour, not drops per minute.

Over 24 hours a patient drank 1,250 mL, received 600 mL intravenously and had 200 mL of soup. Urine output was 900 mL and vomit 350 mL. What is the fluid balance?

• A+1,050 mL
• B+800 mL✓
• C−800 mL
• D+1,150 mL

Answer: Total input = 1,250 + 600 + 200 = 2,050 mL. Total output = 900 + 350 = 1,250 mL. Balance = input − output = +800 mL, a positive balance.

Adrenaline 1 in 1,000 contains 1 mg in 1 mL. An adult with anaphylaxis needs 500 micrograms intramuscularly. What volume is that?

• A0.05 mL
• B5 mL
• C0.5 mL✓
• D1 mL

Answer: 1 mg in 1 mL is 1,000 micrograms in 1 mL, so 500 micrograms is 0.5 mL. This is the adult intramuscular dose in Resuscitation Council UK anaphylaxis guidance, repeated after 5 minutes if there is no improvement.

A syringe driver contains 30 mg of morphine in 24 mL, to run over 24 hours. What rate is the driver delivering per hour?

• A2.5 mg/hour
• B1 mg/hour
• C1.25 mg/hour✓
• D0.8 mg/hour

Answer: 30 mg over 24 hours is 30 ÷ 24 = 1.25 mg per hour (which is also 1 mL per hour of this solution). Checking a driver's rate against the prescription is a routine safety check on every visit.

A patient's fluid restriction is 1.5 litres in 24 hours. By 14:00 they have had 950 mL. How much fluid remains for the rest of the day?

• A650 mL
• B1,050 mL
• C550 mL✓
• D450 mL

Answer: 1.5 litres is 1,500 mL. 1,500 − 950 = 550 mL remaining. Converting the litres to millilitres first avoids the common error of trying to subtract 950 from 1.5 as if the units already matched.

A patient needs 600 mL of feed over 10 hours via a pump. What rate should be set?

• A6 mL/hour
• B50 mL/hour
• C100 mL/hour
• D60 mL/hour✓

Answer: 600 mL ÷ 10 hours = 60 mL per hour. The pump is set in millilitres per hour, so the hours must be used directly rather than converted to minutes.

2,000 mL of fluid is prescribed over 24 hours through a pump. What is the rate, to the nearest whole millilitre per hour?

• A90 mL/hour
• B83 mL/hour✓
• C80 mL/hour
• D120 mL/hour

Answer: 2,000 ÷ 24 = 83.3, which rounds to 83 mL/hour. Setting the pump to 120 mL/hour would deliver the bag in under 17 hours.

1,000 mL is to be given over 6 hours through a giving set delivering 20 drops per mL. What drip rate should be set, to the nearest drop per minute?

• A28 drops/min
• B167 drops/min
• C20 drops/min
• D56 drops/min✓

Answer: Drops per minute = (1,000 × 20) ÷ (6 × 60) = 20,000 ÷ 360 = 55.6, which rounds to 56. 167 is the rate in mL per hour.

A patient weighing 70 kg is prescribed enoxaparin at 1 mg per kg twice daily for a DVT. What is each dose?

• A140 mg
• B35 mg
• C7 mg
• D70 mg✓

Answer: Weight-based dosing: 1 mg × 70 kg = 70 mg per dose, given twice a day. 140 mg is the total for the day, not a single dose.

A patient's total input for the day is 1,800 mL and total output is 1,450 mL. What is the fluid balance?

• A+3,250 mL
• B+450 mL
• C+350 mL✓
• D−350 mL

Answer: Balance = input − output = 1,800 − 1,450 = +350 mL, a positive balance. Adding the two figures together (3,250 mL) is a common slip on a busy chart.

500 mL of an antibiotic infusion is to run over 4 hours via a pump. What rate is set?

• A50 mL/hour
• B125 mL/hour✓
• C100 mL/hour
• D250 mL/hour

Answer: 500 ÷ 4 = 125 mL/hour. Doubling the time to 8 hours would halve the rate to 62.5 mL/hour.

A syringe driver holds 48 mg of a drug in 24 mL to run over 24 hours. How much drug is delivered each hour?

• A0.5 mg
• B2 mg✓
• C1 mg
• D4 mg

Answer: 48 mg over 24 hours is 48 ÷ 24 = 2 mg per hour, delivered as 1 mL per hour of this 2 mg/mL solution.