Diagnostic Procedures Flashcards

A pharmacist reviews a digoxin serum level of 2.4 nmol/L drawn 6 hours post-dose in a 78-year-old woman with CKD (CrCl 22 mL/min) and atrial fibrillation. Her dose was changed 4 days ago. Which interpretation is MOST accurate?

Answer: The level is not at steady state because renal impairment extends digoxin half-life beyond 4 days

Digoxin has a half-life of approximately 36–48 hours in normal renal function, but in severe CKD (CrCl <30 mL/min) the half-life can extend to 4–6 days or longer. Steady state requires 4–5 half-lives. At 4 days post-dose change with a half-life of ~5 days, the patient is only at roughly 50–60% of steady state, making the level an underestimate of the eventual steady-state concentration. Acting on this level risks future toxicity once true steady state is reached.

A drug commonly prescribed for tuberculosis is known to cause false-positive results on which specific diagnostic test, due to its chemical interference with the assay reagent rather than true physiological effect?

Answer: Rifampin causing false-positive urine ketones on nitroprusside-based dipstick tests

Rifampin (rifampicin) is an orange-red pigmented drug that can interfere with colorimetric assays. Specifically, its metabolites can cause false-positive results on nitroprusside-based urine ketone tests due to colour interference. Pyrazinamide (not rifampin) is the TB drug most associated with hyperuricaemia; it does not significantly interfere with the Jaffé creatinine assay. Isoniazid has no clinically significant TSH assay interference, and rifampin does not affect glucose oxidase strips.

A 55-year-old male on long-term lithium therapy for bipolar I disorder presents with a serum lithium of 1.0 mmol/L (drawn 13 hours post-last-dose), polyuria, and a urine osmolality of 145 mOsm/kg even after 8 hours of water deprivation. Administration of exogenous desmopressin (DDAVP) fails to increase urine osmolality above 200 mOsm/kg. Which diagnostic conclusion is BEST supported?

Answer: Nephrogenic diabetes insipidus caused by lithium-induced impairment of renal ADH responsiveness

Lithium's primary mechanism of nephrotoxicity is inhibition of adenylyl cyclase in the collecting duct principal cells, rendering them unresponsive to ADH (vasopressin). The diagnostic hallmark of nephrogenic DI is failure to concentrate urine despite adequate ADH—confirmed here by the failure to respond to exogenous DDAVP (which would correct central DI). In primary polydipsia, urine osmolality rises with prolonged water deprivation. This patient's lithium level is within therapeutic range, arguing against acute toxicity-driven osmotic diuresis.

A 42-year-old woman on valproic acid (VPA) for epilepsy has a total VPA level of 45 mg/L (therapeutic: 50–100 mg/L) but continues to have breakthrough seizures. Her albumin is 22 g/L (normal 35–50 g/L). Which pharmacokinetic concept BEST explains why dose escalation based solely on the total level would be inappropriate?

Answer: Hypoalbuminaemia increases the free fraction of VPA, meaning the free (active) drug concentration may already be therapeutic or supratherapeutic despite low total level

Valproic acid is approximately 90–95% protein-bound (primarily to albumin) at normal albumin levels. With severe hypoalbuminaemia (albumin 22 g/L), the free fraction increases substantially—the total level appears low, but free drug concentration may be adequate or even toxic. Escalating dose based on a low total level in a hypoalbuminaemic patient risks free-drug toxicity. The correct approach is to measure a free VPA level. VPA does not significantly autoiduce CYP2C9, and protein binding does not affect renal clearance of VPA (it is primarily hepatically metabolized).

In interpreting a spot urine albumin-to-creatinine ratio (ACR), which patient characteristic creates the MOST clinically significant false elevation of ACR that could lead to misclassification as moderately increased albuminuria (A2)?

Answer: The patient had vigorous aerobic exercise within 24 hours before specimen collection

Vigorous exercise within 24 hours of urine collection is a well-established cause of transient, physiological albuminuria (functional proteinuria) that can elevate the ACR into the A2 range (30–300 mg/g) without indicating true glomerular disease. CKD staging guidelines explicitly require excluding this confounder before diagnosing albuminuria. A UTI can cause proteinuria through inflammation but also elevates creatinine minimally; it is a confounding factor but exercise-induced elevation is more predictable and clinically important to distinguish from true CKD-associated albuminuria. First-morning voids are actually preferred as they minimize orthostatic/exercise-induced proteinuria.

A hospital pharmacist is consulted regarding a patient on amiodarone (600 mg/day load for 2 weeks, now 400 mg/day for 3 weeks) whose thyroid function tests show: TSH 0.08 mIU/L (low), free T4 28 pmol/L (elevated), free T3 3.2 pmol/L (low-normal). Which interpretation of this thyroid function pattern is MOST consistent with amiodarone's pharmacological mechanism?

Answer: Expected euthyroid pattern from amiodarone inhibiting type 1 deiodinase, causing high T4, low T3 conversion, with TSH lag

Amiodarone potently inhibits type 1 5'-deiodinase (the peripheral enzyme converting T4→T3), causing a characteristic pattern: elevated free T4, low-to-normal T3, and transiently suppressed or low TSH during the first weeks of therapy as the pituitary adapts to changing hormone ratios. This is an expected euthyroid pharmacological effect, not true thyrotoxicosis. Distinguishing this from amiodarone-induced thyrotoxicosis (AIT) requires clinical context—AIT Type I (iodine-driven) and Type II (destructive thyroiditis) both present with true clinical hyperthyroidism with elevated T3, weight loss, and tachycardia. The low T3 in this patient argues against true thyrotoxicosis.