2.6: Physiological Values & Clinical Lab Interpretations

2.6: Physiological Values & Clinical Lab Interpretations

Pharmacists are key clinical stakeholders in reviewing laboratory data to optimise pharmacotherapy, adjust drug doses, and monitor for drug-induced toxicities. For the OPRA exam, a thorough understanding of standard Australian reference ranges, the interpretation of renal and hepatic markers, and high-risk drug monitoring protocols is required.

1. Key Laboratory Reference Ranges (Australian Standards)

In Australian practice, laboratory values are aligned with the Royal College of Pathologists of Australasia (RCPA) reference intervals and the Australian Medicines Handbook (AMH).

2. Interpreting Renal Function for Drug Dosing

Renal impairment alters the pharmacokinetics of many drugs, increasing the risk of accumulation and toxicity.

• eGFR vs. Calculated Creatinine Clearance (CrCl):
  • Modern Australian pathology reports display eGFR (estimated Glomerular Filtration Rate) calculated using the CKD-EPI equation. This is normalized to a standard body surface area (BSA) of 1.73m². eGFR is excellent for staging chronic kidney disease.
  • However, for drug dosing, the Cockcroft-Gault equation is still the clinical standard because drug registration trials and product monographs historically utilized calculated CrCl (in mL/min) to determine dosing adjustments: CrCl (mL/min) = [(140 - age) * weight (kg) * constant] / Serum Creatinine (umol/L) Where the constant is 1.23 for males and 1.04 for females.
  • Dosing Trap: In patients who are elderly, cachectic, or amputees, muscle mass is severely reduced, leading to low serum creatinine. Consequently, eGFR may overestimate renal function. Pharmacists must calculate CrCl using actual or adjusted body weight to prevent overdosing narrow therapeutic index drugs (e.g., digoxin, lithium, direct oral anticoagulants [DOACs]).
• Key Drug Renal Adjustments:
  • Metformin: Maximum dose 1 g daily if CrCl is 30–45 mL/min; contraindicated if CrCl < 30 mL/min due to the risk of metformin-associated lactic acidosis.
  • Direct Oral Anticoagulants (DOACs):
    • Dabigatran: Contraindicated if CrCl < 30 mL/min.
    • Rivaroxaban: Reduce dose from 20 mg to 15 mg daily if CrCl is 15–49 mL/min.
    • Apixaban: Reduce dose from 5 mg twice daily to 2.5 mg twice daily if the patient has a CrCl of 15–29 mL/min, OR if the patient meets at least two of the following criteria: age >= 80 years, body weight <= 60 kg, or serum creatinine >= 133 µmol/L.
  • Gabapentin and Pregabalin: Renally cleared and require progressive dose reductions starting at CrCl < 60 mL/min to prevent neurotoxicity (ataxia, severe sedation, confusion).
  • Allopurinol: Accumulation of its active metabolite, oxypurinol, in renal impairment increases the risk of allopurinol hypersensitivity syndrome (DRESS). Initiate at a lower dose (e.g., 50–100 mg daily) and titrate slowly.

3. Interpreting Liver Function Tests and Drug-Induced Liver Injury (DILI)

Liver function tests are divided into markers of hepatocellular damage, markers of biliary/cholestatic dysfunction, and markers of synthetic function.

A. Hepatocellular vs. Cholestatic Injury Patterns

• Hepatocellular Pattern: Disproportionately high elevation of ALT and AST (often >3 times the upper limit of normal) compared to ALP.
  • Pathophysiology: Direct damage to hepatocytes, causing leakage of intracellular enzymes.
  • Common Causes: Paracetamol toxicity, statins, isoniazid, sodium valproate, amiodarone, NSAIDs.
• Cholestatic Pattern: Disproportionately high elevation of ALP and GGT compared to ALT/AST, often accompanied by elevated bilirubin (leading to pruritus and jaundice).
  • Pathophysiology: Impairment of bile flow or obstruction of the biliary tract.
  • Common Causes: Flucloxacillin (can cause cholestatic hepatitis up to 6 weeks after stopping therapy), amoxicillin-clavulanic acid (Augmentin), erythromycin, oral contraceptives, phenothiazines.
• Isolated GGT Elevation: Suggests microsomal enzyme induction (e.g., by phenytoin, carbamazepine, rifampicin) or chronic alcohol consumption, rather than structural liver disease.

B. Markers of Synthetic Function

Aminotransferases measure cell damage, not function. Liver synthetic capacity is evaluated using:

1. Albumin: Synthesized exclusively by the liver. Since its half-life is long (~20 days), hypoalbuminaemia is a feature of chronic liver disease (e.g., cirrhosis). It is also affected by nutrition and systemic inflammation.
   • Clinical Importance: Hypoalbuminaemia increases the free (active) fraction of highly protein-bound drugs (e.g., phenytoin, warfarin), increasing toxicity risk.
2. INR (International Normalised Ratio): Measures the clotting cascade. Clotting factors (specifically factors II, VII, IX, and X) are synthesized by the liver and have short half-lives (e.g., factor VII half-life is 6 hours). A prolonged INR in the absence of warfarin or DOACs is a sensitive marker of acute, severe liver injury.

4. Haematological Monitoring Protocols for High-Risk Drugs

A. Clozapine Monitoring Protocol (Australia)

Clozapine, an atypical antipsychotic for treatment-resistant schizophrenia, is associated with a risk of severe neutropenia and life-threatening agranulocytosis (ANC < 0.5 × 10⁹/L).

• In Australia, patients, prescribers, and pharmacies must register with a national registry (e.g., ClopineCentral). Dispensing clozapine is illegal without a valid, recent Full Blood Count (FBC).
• Monitoring Frequency: Weekly for the first 18 weeks, and monthly thereafter if blood counts remain stable.
• The Traffic Light System:
  • Green: WCC >= 3.5 × 10⁹/L AND ANC >= 2.0 × 10⁹/L. Action: Continue standard monitoring and dispense.
  • Amber: WCC 3.0 – 3.5 × 10⁹/L OR ANC 1.5 – 2.0 × 10⁹/L. Action: Continue clozapine, but increase FBC monitoring frequency to twice weekly until values stabilize or return to Green.
  • Red: WCC < 3.0 × 10⁹/L OR ANC < 1.5 × 10⁹/L. Action: Immediately stop clozapine. Do not re-initiate. Refer the patient to a haematologist. Monitor blood counts daily.

B. Methotrexate Monitoring Protocol

Oral methotrexate is prescribed once weekly for autoimmune conditions like rheumatoid arthritis and psoriasis.

• Safety Alert: Accidentally taking methotrexate daily instead of weekly causes rapid bone marrow suppression, manifesting as severe leucopenia and thrombocytopenia, along with gastrointestinal ulceration.
• Monitoring Requirements: FBC, LFTs, and renal function must be checked before initiating treatment, every 1–2 weeks for the first few weeks, and then every 2–3 months once stable.
• Folic Acid Co-prescribing: Standard practice in Australia is to co-prescribe folic acid (usually 5 mg weekly) to be taken on a different day from the methotrexate dose. This mitigates side effects like mucosal ulcers, alopecia, and mild myelosuppression without compromising methotrexate's therapeutic efficacy.