A 28-year-old woman with newly diagnosed pulmonary TB is about to start anti-TB therapy. She reports taking oral contraceptives for the past 2 years. Before initiating treatment, which investigation is most appropriate to assess baseline organ function and guide drug dosing?

## Baseline Investigations Before Anti-TB Therapy Initiation ### Clinical Context Before starting first-line anti-TB drugs (HRZE), baseline organ function assessment is essential because: - **Hepatotoxicity**: INH, RIF, PZA are hepatotoxic - **Nephrotoxicity**: Ethambutol can cause renal impairment - **Neurotoxicity**: INH causes peripheral neuropathy; ethambutol causes optic neuritis - **Drug interactions**: RIF induces oral contraceptive metabolism (relevant in this case) ### Why Serum Creatinine, LFTs, and Visual Acuity Testing **Key Point:** Baseline renal and hepatic function + visual acuity are mandatory before first-line TB therapy to detect pre-existing organ dysfunction and establish a baseline for monitoring toxicity. ### Rationale for Each Component | Baseline Test | Why Essential | Abnormality Detected | |---|---|---| | **Serum creatinine** | Assess renal function before ethambutol | Pre-existing renal disease; guides EMB dosing | | **Liver function tests (AST, ALT, bilirubin)** | Assess hepatic reserve before INH/RIF/PZA | Cirrhosis, chronic hepatitis, jaundice; predicts DILI risk | | **Visual acuity testing (Snellen chart)** | Baseline before ethambutol (optic neuritis risk) | Establishes baseline; early detection of EMB toxicity | | **Sputum smear / culture** | Confirms diagnosis, assesses infectivity | Baseline for monitoring treatment response | **High-Yield:** The classic triad of baseline investigations = **Renal function + Hepatic function + Visual acuity**. ### Special Consideration: Oral Contraceptive Use **Clinical Pearl:** Rifampicin is a potent CYP3A4 inducer and reduces oral contraceptive efficacy by 40–50%. This patient should be counseled about contraceptive failure and offered alternative contraception (barrier method, IUD, or progesterone-only pills). However, this is a counseling point, not a baseline investigation. ### Mnemonic for Baseline TB Investigations **"LIVER + EYES + KIDNEYS"** - **L**iver function (AST, ALT, bilirubin) → INH/RIF/PZA hepatotoxicity - **I**nvestigate renal function (creatinine) → EMB nephrotoxicity - **V**isual acuity → EMB optic neuritis - **E**yes baseline → early detection of color blindness - **R**enal baseline → dose adjustment if CKD - **K**eep baseline for comparison → monitor for drug-induced toxicity ### Comparison with Other Investigations | Investigation | Role | Appropriateness for Baseline | |---|---|---| | **Serum Cr + LFTs + visual acuity** | Assess organ function before hepatotoxic/nephrotoxic drugs | ✓ Mandatory | | **CXR + sputum smear** | Diagnostic confirmation, assess extent | Already done (diagnosis confirmed); not for baseline organ function | | **Blood glucose + electrolytes** | Screen for diabetes, hypokalemia | Not routine; only if symptoms or risk factors present | | **Audiometry + renal ultrasound** | Assess hearing, renal anatomy | Not routine baseline; audiometry is for aminoglycoside toxicity (second-line TB drugs) | **Warning:** Do NOT start HRZE without baseline LFTs and visual acuity. Delayed detection of drug-induced liver injury or optic neuritis can lead to irreversible organ damage.