Weight-based and paediatric doses

The milligram-per-kilogram rule assumes that dose rises in direct proportion to body mass, and that assumption is an approximation rather than a law. A newborn and an adolescent weighing the same number of kilograms do not clear a drug at the same rate, because the organs that handle it are at different stages of development. In the first months of life, hepatic enzyme pathways are still maturing and glomerular filtration is low, so drugs eliminated by the kidney or by the liver can accumulate and reach toxic concentrations at what looks like a correct mg/kg dose. Near the other end of childhood, metabolic rate per kilogram falls, and a proportional calculation can overshoot. Because of this, some drugs — cytotoxics and several paediatric medicines — are ordered by body surface area instead, in milligrams per square metre (mg/m²), calculated from height and weight with a nomogram. Surface area tracks metabolic activity and blood flow better than mass does. Two further ideas belong here. First, the effective dose depends not only on absorption and distribution but also on the drug’s half-life, which sets how often it must be given. Second, neonates and infants frequently need their own concentration of a liquid medicine, because a volume that is correct for an older child may be too small to measure accurately in a newborn. The nursing consequence is direct: mg/kg is the common rule, not the only one, and the order, the age and the concentration must be read together before any arithmetic begins.

A maximum dose is the ceiling that proportional arithmetic cannot see. It exists because every drug has a therapeutic index — the distance between the dose that helps and the dose that harms. For a drug with a narrow index, such as digoxin, gentamicin or paracetamol, that distance is small, and a calculation that is arithmetically perfect can still sit above the ceiling. Two kinds of limit are quoted: a maximum single dose, which caps one administration, and a maximum daily dose, which caps the total across 24 hours. A heavy child can pass the single-dose limit while still being inside the daily limit, and a child given a drug every four hours can pass the daily limit even though each individual dose looks small. This is why a calculated total must be compared against both, and why the notes put “check recommended or maximum dose limits where applicable” in the middle of the verification list. The nursing process that makes the check real is the independent double check: two nurses calculate separately, without one reading the other’s figure first, and then compare. Independence is the whole point, because a shared error is not detected by a second person copying the first answer. Paediatric errors are also known for a particular shape — the tenfold error, produced by a misplaced decimal point or a transposed digit, which is why every calculation is written with its units and not carried in the head. Documentation closes the loop: the weight used, the dose calculated, the volume drawn and the time given are recorded, so that a later discrepancy can be traced rather than guessed.

After the arithmetic comes a decision the formula cannot make: how the volume is rounded, and whether the result can be measured at all. A calculated 0.63 mL is not a volume a nurse draws up by eye; it must be rounded according to the ward policy and the graduations of the available syringe. Different syringes offer different precision — a 1 mL syringe is usually graduated in 0.01 mL increments, a 5 mL syringe in 0.2 mL, and a large syringe in whole millilitres — so the same calculation can be measurable in one syringe and not in another. Rounding is never a free choice, and it is never upward for convenience: rounding a dose up “to make it easier” increases the amount the child receives, while rounding a tiny paediatric volume down can remove most of the dose. Where the rounded volume falls outside the tolerances the policy allows, the correct move is not to guess but to ask the pharmacy to dispense a more suitable concentration or a smaller syringe. Two practical points follow. Liquid suspensions must be shaken before measuring, because the drug settles and the first millilitres poured may not carry the labelled strength. And a concentration should be confirmed on the container itself, not from memory of the bottle that usually sits on that shelf. When the syringe cannot deliver the calculated volume accurately, the prescription is not wrong and the arithmetic is not wrong; the preparation is simply unsuitable, and pharmacy is the department that can change it.