14.4 Safe Prescribing Principles & Antimicrobial Stewardship

Safe Prescribing Principles & Antimicrobial Stewardship

Prescribing in United Kingdom clinical practice is governed by professional standards established by the General Medical Council (GMC) in Good Medical Practice and the British National Formulary (BNF). Safe prescribing requires meticulous chart documentation, dosage adjustment in organ failure, identification of Cytochrome P450 interactions, pharmacovigilance under the MHRA Yellow Card scheme, and adherence to Antimicrobial Stewardship protocols.

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Standards of Safe Prescribing in NHS Practice

Prescription errors represent one of the most common causes of preventable iatrogenic patient harm. Whether utilizing electronic prescribing systems (EPMA) or conventional paper inpatient medication charts (Kardex), every legal prescription must conform to standard BNF rules:

• Patient Identification: Must state patient full name, date of birth, unique hospital number (and NHS number), and inpatient ward.
• Mandatory Allergy Status: The allergy box must be completed prior to any drug administration. Document the specific allergen, the precise nature of the reaction (e.g., immediate anaphylaxis vs mild gastrointestinal upset), date of occurrence, and prescriber signature. If no allergies exist, prescribers must actively write 'NKDA' (No Known Drug Allergies) and sign; leaving the allergy box blank is a serious prescribing violation.
• Generic Drug Naming: Drugs must be prescribed by their approved non-proprietary generic names in clear, legible capital letters (e.g., write 'PARACETAMOL', never 'Panadol').
  • Exceptions to Generic Rule: Biological medicines (insulins, monoclonal antibodies, biosimilars) and medications where bioavailability differences between formulations produce clinical instability (e.g., lithium carbonate, modified-release theophylline, diltiazem, tacrolimus, ciclosporin).
• Prescriber Accountability: Every prescription entry requires a clear signature, legible printed name, contact bleep or telephone number, and GMC professional registration number.

Eliminating Ambiguity: Decimal Point & Unit Rules

Ambiguous numerical notation is a primary driver of tenfold or hundredfold dosage errors:

• Never Use Trailing Zeros: Write 1 mg, NEVER 1.0 mg. If the decimal point is obscured by chart folding or faint printing, '1.0 mg' will be misinterpreted and administered as 10 mg (a fatal tenfold overdose).
• Always Use Leading Zeros: Write 0.5 mL, never .5 mL. If the decimal point is missed, '.5 mL' becomes 5 mL.
• Prescribing in Micrograms & Nanograms: Doses under 1 milligram must be written in full in micrograms or nanograms (e.g., write 500 micrograms, NEVER 0.5 mg; write 100 micrograms, never 0.1 mg). Avoid ambiguous abbreviations such as 'mcg' or 'µg'.
• Prescribing Units: Never abbreviate units as 'U' or 'IU' (e.g., in insulin or heparin prescriptions). A handwritten 'U' is readily misread as a zero ('0') or a '4', turning '10 U' into '100' units—causing catastrophic hypoglycaemic brain death. Always write the word 'units' in full.

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Prescribing in Organ Impairment

1. Renal Impairment & Haemodynamic Drug Toxicity

Renal drug elimination depends on glomerular filtration and tubular secretion. In the UK, chronic kidney disease (CKD) is staged using estimated Glomerular Filtration Rate (eGFR via the CKD-EPI equation). However, for narrow therapeutic index drugs and DOACs, the BNF and MHRA mandate calculating Creatinine Clearance (CrCl) using the Cockcroft-Gault formula: $\text{CrCl (mL/min)} = \frac{(140 - \text{Age}) \times \text{Weight (kg)} \times 1.23\text{ [1.04 for females]}}{\text{Serum Creatinine (micromol/L)}}$

• Renally Eliminated Drugs Requiring Dose Reduction or Cessation:
  • Metformin: Exclusively renally cleared; accumulates in renal failure, predisposing to fatal lactic acidosis. Review and halve dose if eGFR <45 mL/min; discontinue if eGFR <30 mL/min or in acute dehydration, sepsis, or shock.
  • Direct Oral Anticoagulants (DOACs): Accumulate in renal impairment, increasing major haemorrhage. Dabigatran (80% renal clearance) is strictly contraindicated if CrCl <30 mL/min. Apixaban, rivaroxaban, and edoxaban require dose reduction if CrCl 15–50 mL/min and are avoided if CrCl <15 mL/min.
  • Low Molecular Weight Heparins (LMWH): Enoxaparin and dalteparin accumulate in severe renal failure. If CrCl <30 mL/min, reduce therapeutic dosage by 50%, monitor anti-Factor Xa levels, or convert to intravenous Unfractionated Heparin (UFH) monitored with APTT ratios.
  • Other High-Risk Cleared Agents: Digoxin, aminoglycosides (gentamicin), glycopeptides (vancomycin), methotrexate (severe marrow aplasia), atenolol, and lithium.

2. Hepatic Impairment

The liver governs drug clearance through phase I (oxidation via Cytochrome P450) and phase II (conjugation) metabolism, alongside synthesizing plasma proteins (albumin and clotting factors).

• High First-Pass Clearance Drugs: In severe cirrhosis or portosystemic shunting, systemic bioavailability of drugs that normally undergo substantial first-pass hepatic extraction rises exponentially. Agents such as morphine, propranolol, labetalol, and verapamil achieve massive plasma concentrations from standard oral doses, causing profound respiratory arrest or cardiogenic shock.
• Hypoalbuminaemia & Protein Binding: Albumin synthesis falls in chronic liver disease. Highly protein-bound drugs (e.g., phenytoin, warfarin) have a higher unbound (free, pharmacologically active) fraction, increasing toxicity despite 'normal' total serum levels.
• Precipitation of Hepatic Encephalopathy: Cirrhotic patients exhibit extreme cerebral sensitivity to central nervous system depressants. Opioids, benzodiazepines, and sedating antihistamines precipitate acute hepatic encephalopathy and coma.
• Fluid Retention: NSAIDs inhibit renal prostaglandins and trigger massive sodium and water retention in cirrhotic patients, exacerbating ascites, peripheral edema, and inducing hepatorenal syndrome.
• Paracetamol Safety in Liver Disease: Contrary to common misconception, paracetamol remains the analgesic of choice in stable chronic liver disease; however, the maximum daily dose must be reduced to 2 g to 3 g per 24 hours (e.g., 500 mg QDS) to prevent toxic glutathione depletion.

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Cytochrome P450 Drug-Drug Interactions

The hepatic microsomal Cytochrome P450 (CYP450) isoenzyme superfamily (predominantly CYP3A4, CYP2D6, CYP2C9, CYP2C19, CYP1A2) is the primary site of oxidative drug biotransformation. Interactions occur via enzyme induction or enzyme inhibition:

1. Cytochrome P450 Enzyme Inducers

Enzyme inducers accelerate CYP gene transcription and protein synthesis, expanding metabolic capacity over 1 to 2 weeks. Substrate drugs are degraded much faster, resulting in subtherapeutic plasma concentrations and clinical treatment failure.

• Inducer Mnemonic (CRAP GPS):

  • C — Carbamazepine
  • R — Rifampicin (most potent known inducer)
  • A — Alcohol (chronic regular abuse)
  • P — Phenytoin
  • G — Griseofulvin
  • P — Phenobarbital
  • S — St John's Wort (Hypericum perforatum, herbal remedy)

• Critical Clinical Consequences of Enzyme Induction:

  • Warfarin: Rapid metabolic clearance -> INR drops precipitously below therapeutic range -> thromboembolic stroke / valve thrombosis.
  • Combined Hormonal Contraceptives: Estrogen and progestogen are cleared rapidly -> contraceptive failure and unintended pregnancy (patients on enzyme inducers must switch to an intrauterine device [IUD/IUS] or medroxyprogesterone injection [Depo-Provera]).
  • Antiretrovirals, DOACs, & Corticosteroids: Plasma concentrations plummet, triggering viral breakthrough, thrombosis, or Addisonian crisis.

2. Cytochrome P450 Enzyme Inhibitors

Enzyme inhibitors compete for or directly inactivate CYP catalytic sites, exerting an almost instantaneous inhibitory effect (within 24–48 hours). Substrate drugs fail to clear, leading to drug accumulation and severe clinical toxicity.

• Inhibitor Mnemonic (SICKFACES.COM):

  • S — Sodium valproate
  • I — Isoniazid
  • C — Cimetidine
  • K — Ketoconazole / Fluconazole (and other azole antifungals)
  • F — Fluoxetine / Fluvoxamine (SSRIs)
  • A — Alcohol (acute binge drinking)
  • C — Chloramphenicol
  • E — Erythromycin / Clarithromycin (macrolide antibiotics)
  • S — Sulfonamides / Co-trimoxazole
  • C — Ciprofloxacin (fluoroquinolones)
  • O — Omeprazole
  • M — Metronidazole
  • Grapefruit Juice (potent intestinal CYP3A4 inhibitor)

• Critical Clinical Consequences of Enzyme Inhibition:

  • Warfarin + Macrolides / Metronidazole / Ciprofloxacin: Massive reduction in warfarin metabolism -> INR surges to >8.0 -> catastrophic spontaneous intracranial or gastrointestinal haemorrhage.
  • Statins (Simvastatin/Atorvastatin) + Macrolides (Clarithromycin/Erythromycin): Inhibition of CYP3A4 causes statin accumulation -> extensive muscle necrosis, myoglobinuria, and acute tubular necrosis (severe rhabdomyolysis). Statins must be suspended during macrolide therapy.
  • Theophylline + Ciprofloxacin: Toxic theophylline accumulation -> fatal cardiac arrhythmias and intractable seizures.

Other Critical Pharmacodynamic Interactions

• Serotonin Syndrome: Co-administration of multiple serotonergic agents (e.g., SSRIs + MAOIs, triptans, tramadol, linezolid, St John's Wort). Overstimulation of central 5-HT₁A and 5-HT₂A receptors causes a classic triad: neuromuscular hyperactivity (spontaneous or inducible clonus, hyperreflexia, tremor), autonomic instability (hyperthermia, diaphoresis, tachycardia, labile BP), and altered mental status (agitation, delirium).

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Adverse Drug Reactions & The MHRA Yellow Card Scheme

Adverse Drug Reactions (ADRs) are classified using the Rawlins and Thompson framework:

The MHRA Yellow Card Scheme

The Medicines and Healthcare products Regulatory Agency (MHRA) and the Commission on Human Medicines (CHM) operate the Yellow Card Scheme for pharmacovigilance and spontaneous ADR reporting.

• Black Triangle ($\blacktriangledown$) Medicines: Designates newly licensed drugs and vaccines subjected to intensive post-marketing monitoring for 2 to 5 years. Prescribers and patients must report ALL suspected adverse reactions, no matter how minor or well-recognized.
• Established Drugs: Healthcare professionals should report all serious suspected adverse reactions (reactions that are fatal, life-threatening, disabling, cause congenital malformation, or require hospitalisation/prolonged bed stay), even if the reaction is already documented in the BNF.

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Antimicrobial Stewardship & Clostridioides Difficile

Antimicrobial Stewardship (AMS) encompasses systematic organizational and clinical interventions designed to optimize antimicrobial selection, dosing, route, and duration, curbing the emergence of multidrug-resistant pathogens (e.g., MRSA, ESBL, CPE) and preventing secondary infections.

UKHSA Protocol: "Start Smart - Then Focus"

1. Start Smart (At Initial Presentation):
   • Take appropriate microbiological cultures (blood cultures, urine, sputum, wound swabs, CSF) prior to commencing antimicrobials, provided this does not cause life-threatening delays in severe sepsis (where antibiotics must be administered within 1 hour).
   • Initiate empirical therapy compliant with local evidence-based trust antimicrobial guidelines.
   • Document on the drug chart and in medical notes: precise clinical indication, duration/review date, dose, and route.
2. Then Focus (Mandatory Clinical Review at 48 to 72 Hours):
   • At 48–72 hours, review all antimicrobial prescriptions alongside microbiological culture and sensitivity results. Formulate one of five definitive actions:
     1. Stop: Cease antibiotics if clinical evaluation and biomarkers (CRP, procalcitonin) indicate no bacterial infection.
     2. Switch (IVOST): Switch from intravenous to oral therapy if the patient is improving, afebrile for 24–48 hours, and tolerating oral intake.
     3. Change (De-escalate): Narrow the antimicrobial spectrum based on microbiological sensitivity results (e.g., switch from meropenem to oral amoxicillin).
     4. Continue: Maintain treatment with a clear, documented stop date and subsequent review milestone.
     5. OPAT: Refer for Outpatient Parenteral Antimicrobial Therapy.

Minimizing the "4C" Antibiotics & Clostridioides Difficile Infection

Broad-spectrum antimicrobials decimate commensal colonic microflora (especially anaerobic bacteroidetes), allowing spore-forming Clostridioides difficile to proliferate and produce enterotoxin A and cytotoxin B. Four antibiotic classes—the "4C" antibiotics—confer the highest relative risk for precipitating C. difficile colitis and must be strictly curtailed:

1. Cephalosporins (e.g., ceftriaxone, cefotaxime, cefuroxime)
2. Co-amoxiclav (amoxicillin-clavulanic acid)
3. Clindamycin (historically highest individual risk)
4. Ciprofloxacin (and all fluoroquinolones: levofloxacin, moxifloxacin)

Diagnosis & Management of C. Difficile Colitis (NICE NG199)

• Clinical Presentation: Watery, foul-smelling diarrhea (≥3 unformed stools in 24 hours), lower abdominal cramping, leukocytosis, and pyrexia.
• Severity Stratification:
  • Mild / Moderate: Normal white cell count ($\text{WBC} <15 \times 10^9\text{/L}$), normal serum creatinine (<50% rise above baseline).
  • Severe: $\text{WBC} >15 \times 10^9\text{/L}$, OR acute serum creatinine elevation (>50% above baseline), OR temperature >38.5°C, OR severe abdominal signs.
  • Life-Threatening: Hypotension, septic shock, paralytic ileus, toxic megacolon, or colonic perforation.
• Treatment Protocol (NICE NG199 Guidelines):
  • First Episode (Any Severity, Not Life-Threatening): Oral vancomycin 125 mg four times daily (QDS) for 10 days is first-line; oral fidaxomicin 200 mg twice daily (BD) for 10 days is second-line if vancomycin is ineffective (NICE NG199).
  • Recurrence: within 12 weeks of symptom resolution, use fidaxomicin; after 12 weeks, vancomycin or fidaxomicin. Seek specialist advice for further recurrences (for example faecal microbiota transplant).
  • Important Clinical Pearl: Oral vancomycin is not absorbed systemically; it acts purely intraluminally within the colonic bowel lumen. Intravenous vancomycin is completely ineffective for treating C. difficile infection because it does not penetrate into the colonic lumen. Oral metronidazole is no longer recommended first-line due to inferior cure and higher recurrence rates.
  • Life-Threatening / Complicated Disease: Oral vancomycin (up to 500 mg QDS via nasogastric tube if ileus) PLUS Intravenous Metronidazole 500 mg three times daily (TDS) alongside immediate emergency colorectal surgical consultation (subtotal colectomy or diverting loop ileostomy with colonic lavage).