Drugs Used as Diagnostic Agents

Drugs Used as Diagnostic Agents

1. Definition, scope and the distinctive pharmacology of a diagnostic agent

• A diagnostic agent is a substance administered not to modify disease but to generate information — to make a structure visible, to unmask a latent functional abnormality, or to quantify the capacity of an organ — and its entire risk–benefit calculus therefore differs from that of a therapeutic drug, because the recipient derives no direct therapeutic gain from the exposure itself. (DiPiro 12e Ch.e60, pp.12–13)
• Diagnostic agents divide pharmacologically into three mechanistically distinct families: (a) contrast and tracer agents, whose action is physical (x-ray attenuation, proton relaxivity, gamma/positron emission, acoustic backscatter) rather than receptor-mediated; (b) provocative/functional agents, which act at conventional receptors or enzymes to produce a measurable physiological stress or hormonal response; and (c) clearance and permeability markers, which are pharmacologically inert and are used purely because their disposition is predictable. (DiPiro 12e Ch.e29, p.20; Ch.e60, pp.7–8)
  • The first family is dosed in grams (iodine, barium) or microcuries/millicuries (radionuclides) and its adverse effects are largely idiosyncratic or osmotic; the second is dosed in conventional pharmacological terms and its adverse effects are exaggerated on-target actions; the third should ideally have no pharmacological action at all.
• Ideal properties of a clearance marker, stated formally for GFR measurement, generalise to all inert diagnostic markers: freely filtered, not secreted, not reabsorbed, not metabolised within the organ studied, and must not itself alter the function being measured. (DiPiro 12e Ch.e60, p.7)
  • Under those conditions measured GFR equals the renal clearance of the marker: GFR = renal CL = A_e/AUC_0-t, where A_e is amount excreted in urine over time t. (DiPiro 12e Ch.e60, pp.7–8)
  • At steady state during continuous infusion this simplifies to GFR = A_e/(C_ss × t); with plasma-clearance-only methods, CL = infusion rate/C_ss or CL = dose/AUC_0-∞ after a single IV bolus. (DiPiro 12e Ch.e60, p.8)
  • ⚠ Method-dependence is quantified: plasma-clearance methods yield values 10–15% higher than the same marker measured by urine collection — a systematic, not random, discrepancy that must be declared whenever GFR values from different laboratories are compared. (DiPiro 12e Ch.e60, p.8)
• Provocative testing is deliberate iatrogenesis under control. Bronchial provocation exists to demonstrate hyperresponsiveness by causing bronchoconstriction; cardiac stress testing exists to demonstrate ischaemia by causing a supply–demand mismatch; the cosyntropin test exists to demonstrate adrenal reserve by delivering a supraphysiological ACTH stimulus. Every such test therefore carries a defined contraindication list that is essentially a list of patients in whom the induced abnormality cannot be tolerated. (DiPiro 12e Ch.e43, p.8; G&G 14e Ch.50, p.1007)
• The diagnostic and therapeutic use of the same molecule differ by orders of magnitude in dose, which is the single most examinable pharmacological feature of this topic. Radioactive iodine is the archetype: diagnostic dose 25–100 µCi (tracer only, no follicular damage), therapeutic dose 3–6 mCi for hyperthyroidism — a ~50–100-fold separation, with much higher doses again for metastatic thyroid carcinoma. (KDT 8e Ch.18, p.278)
• Renal functional reserve (RFR) is the conceptual bridge between provocative and clearance testing: an oral or IV protein load raises GFR by as much as 50% in normal subjects through renal vasodilatation and increased renal plasma flow, and the loss of that reserve unmasks subclinical nephron loss — explicitly analogous to a cardiac stress test, where resting function is normal and only exertion reveals the lesion. (DiPiro 12e Ch.e60, pp.3, 7)
  • Corollary and a real source of error: dietary protein status alters measured GFR, so vegetarians have lower GFR at identical renal mass and every mGFR must be interpreted against the patient's protein intake at the time of study. (DiPiro 12e Ch.e60, p.7)
• Ordering discipline is itself part of the pharmacology. Before any test involving ionising radiation, the likelihood that the result will change management must be established; "routine" follow-up scans in asymptomatic individuals are to be avoided, and laboratories operate under the ALARA (as low as reasonably achievable) principle. (Harrison 22e Ch.248, p.1883)