Prescription and Dosage Management

Many pediatric drugs are dosed by weight: dose (mg) = dose per kilogram (mg/kg) × weight (kg). Example: amoxicillin for a child with otitis media = 25 mg/kg. If the child weighs 20 kg, dose = 25 × 20 = 500 mg. Always convert pounds to kg: divide by 2.2 (or multiply by 0.45). For IV drips in children, calculate mg/kg, then determine the volume to give based on the concentration of the drug. Example: epinephrine in anaphylaxis = 0.01 mg/kg IM. For a 30 kg child, dose = 0.01 × 30 = 0.3 mg. If you have a 1 mg/mL concentration, draw up 0.3 mL. Always double-check calculations and have another provider verify for high-risk drugs (insulin, opioids, chemotherapy).

TDM is monitoring blood levels of certain drugs to ensure they are in the therapeutic range—high enough to work, low enough to be safe. Drugs requiring TDM include vancomycin (antibiotic; maintain 15–20 mcg/mL peak, 10–15 trough), digoxin (heart drug; 0.5–2.0 ng/mL), warfarin (anticoagulant; monitor INR instead of drug level), lithium (psychiatric; 0.6–1.2 mmol/L), theophylline (asthma; 10–20 mcg/mL). Blood samples are timed: for vancomycin, draw trough before the next dose (steady state = 3–4 days) and peak 30–60 minutes after infusion ends. Abnormal levels guide dose adjustments: if vancomycin trough is 8 mcg/mL (too low), increase the dose or infusion rate; if it is 25 mcg/mL (too high), hold a dose or reduce the rate. Kidney function and liver function affect drug clearance, so renal dosing is essential for renally cleared drugs.

Kidney function is measured by creatinine clearance (CrCl) or glomerular filtration rate (GFR). The Cockcroft-Gault equation estimates CrCl: CrCl = (140 − age) × weight (kg) × (0.85 if female) / (72 × creatinine). Normal CrCl ≥90 mL/min. Stage 3a CKD = 45–59, Stage 3b = 30–44, Stage 4 = 15–29, Stage 5 (ESRD) = <15. Many drugs require dose reduction or interval extension in kidney disease: adjust the dose (give less), extend the interval (give it less often), or both. Example: gentamicin (antibiotic) normal dose = 5 mg/kg IV every 8 hours. In Stage 4 CKD (GFR 20 mL/min), reduce to 5 mg/kg every 24–36 hours instead. Use a renal dosing guide or pharmacy consult if unsure; underdosing antibiotics fails treatment, but overdosing causes toxicity.

A drug interaction occurs when one drug affects how another drug works (increases effect, decreases effect, or causes toxicity). Example: warfarin + aspirin = increased bleeding risk. NSAIDs + ACE inhibitors = kidney damage. Clarithromycin (antibiotic) + simvastatin (cholesterol) = muscle breakdown (rhabdomyolysis). Screen for interactions by: (1) Asking the patient their full medication list (don't forget supplements, OTC, herbals). (2) Using a drug interaction checker (Medscape, EPCS, pharmacy system). (3) Knowing high-risk combinations (warfarin + NSAIDs, SSRIs + MAOIs, statins + fibrates). If a major interaction exists, consult the pharmacist or choose a different drug. Minor interactions may be monitored and are sometimes acceptable. Always document why a patient is on multiple interacting drugs—sometimes the benefit outweighs the risk.

Half-life (t½) is the time it takes for the blood concentration to drop to half. After one half-life, 50% of the drug remains; after two half-lives, 25%; after three, 12.5%; after five, <5% (clinically negligible). Steady state is reached after ~5 half-lives, when the amount given per dose equals the amount cleared, so levels stay constant. Loading dose is a larger initial dose to reach therapeutic levels faster (instead of waiting 5 half-lives). Example: digoxin has a long half-life (~40 hours). To reach therapeutic level slowly = wait ~200 hours (weeks). To reach it immediately = give a loading dose. In an emergency (atrial fibrillation), you give a loading dose; in chronic care, you give a maintenance dose and wait for steady state. Understand t½ for your drugs so you know when to expect peak effect and when to recheck levels.

The five rights: (1) Right patient—verify ID with two identifiers (name + date of birth), not room number. (2) Right drug—confirm the drug name and that the patient has no allergy. (3) Right dose—calculate correctly, verify with another provider for high-risk drugs. (4) Right route—IV, IM, oral, etc. Some drugs are only safe by one route; methotrexate IV is OK, but intrathecal (into spinal fluid) is fatal in high doses. (5) Right time—give at scheduled intervals; timing matters for antibiotics (every 6 hours means every 6 hours, not when convenient). Many institutions now add two more: Right documentation (chart it immediately) and Right evaluation (monitor for effect and side effects). Following these five rights catches 95%+ of medication errors. Errors occur when providers cut corners (verify 'later,' calculate 'in your head,' give 'close enough' doses). Never skip these steps; they are the barrier between safety and harm.

Toxicity signs vary by drug: (1) Acetaminophen—nausea, vomiting, right upper abdominal pain, jaundice, liver failure (within 48–72 hours if overdose). (2) Digoxin—nausea, vomiting, arrhythmias, visual changes ('yellow haze'), confusion. (3) Vancomycin—tinnitus, hearing loss, red-man syndrome (flushing, itching from rapid infusion). (4) NSAIDs—GI bleed (black stools, abdominal pain), acute kidney injury. (5) ACE inhibitors—cough, angioedema (face swelling), hyperkalemia. If toxicity is suspected: STOP the drug, notify the provider, draw a drug level if available, monitor vital signs and organ function (renal, liver), and treat specific toxicity (activated charcoal for acetaminophen OD, digoxin-specific antibody for digoxin toxicity, etc.). Prevention is better than treatment: verify doses, monitor levels, and educate patients on not exceeding OTC drug limits (acetaminophen max 4 g/day).