Percentage Concentrations and Ratio Solutions: CBT Practice

Percentages and ratios are the strengths that don’t say “mg” anywhere on the label. The CBT tests whether you can recover the milligrams from them, because in an emergency that’s exactly the conversion you’ll be doing while someone waits.

Two definitions carry the whole chapter.

Percentage (w/v): grams per 100 mL. Sodium chloride 0.9% has 0.9 g of salt in every 100 mL. Scale from there: 500 mL holds 4.5 g, a litre holds 9 g. Glucose 5% is 5 g per 100 mL, so 50 g in a litre bag.

Ratio: grams in the stated volume. 1:1000 means 1 g in 1,000 mL. Convert once: 1,000 mg in 1,000 mL is 1 mg/mL. 1:10,000 means 1 g in 10,000 mL, so 0.1 mg/mL.

Everything else is multiplication.

The adrenaline pair

The two adrenaline presentations are the highest-stakes example of ratio strengths, and the exam treats them accordingly.

• 1:1000 (1 mg/mL): the IM anaphylaxis ampoule. Adult dose 0.5 mg = 0.5 mL.
• 1:10,000 (0.1 mg/mL): the 10 mL pre-filled syringe for IV use in cardiac arrest. The full syringe is 1 mg.

Both presentations contain 1 mg in their standard packaging. What differs is the concentration, and therefore the route it’s safe by. Giving the 1:1000 strength IV instead of IM is a recognised fatal error, which is why questions about these two strengths keep appearing in every UK nursing exam. Learn the pair as a pair.

Working at speed

Convert the strength to mg per mL first, then forget the percentage ever existed:

• 0.9% saline → 9 mg/mL.
• 5% glucose → 50 mg/mL.
• 1% anything → 10 mg/mL. So 2% lidocaine is 20 mg/mL, and 5 mL of it is 100 mg.

That last derivation, 1% = 10 mg/mL, is the single most useful fact in this chapter. Any percentage the exam invents, multiply by 10 and you have mg per mL.

Wrong options in this topic are nearly always the right digits with the decimal point misplaced. So a slow, written derivation beats a fast mental one: 2% → 2 g/100 mL → 2,000 mg/100 mL → 20 mg/mL. Four short steps you can see, instead of one leap you can’t check.

The five questions below include both adrenaline strengths. After this, Part A moves from concentrations to bookkeeping: fluid balance.