A 28-year-old woman with a diagnosis of Bipolar Disorder II (confirmed by 2 prior hypomanic episodes and recurrent major depressive episodes) presents for routine follow-up. She is currently euthymic on lithium monotherapy. Before initiating long-term lithium maintenance, which investigation is most appropriate to establish baseline renal and cardiac safety?

Explanation

## Baseline Investigations Before Lithium Initiation ### Lithium: Mechanism and Toxicity Profile **Key Point:** Lithium is the gold-standard mood stabilizer for Bipolar Disorder I and II, but it has a narrow therapeutic window (0.6–1.2 mEq/L) and significant organ toxicity risk. Baseline investigations are mandatory before starting therapy. **High-Yield:** Lithium is filtered by the glomerulus and reabsorbed in the proximal tubule. Chronic lithium use causes nephrogenic diabetes insipidus (NDI) and progressive chronic kidney disease (CKD). Lithium also prolongs the QT interval and can cause arrhythmias. ### Why Serum Creatinine, eGFR, and ECG Are the Investigation of Choice **Clinical Pearl:** These three investigations establish baseline renal and cardiac function—the two organ systems most vulnerable to lithium toxicity. 1. **Serum creatinine and eGFR:** - Assess baseline glomerular filtration rate - Lithium is contraindicated if eGFR <30 mL/min/1.73m² (absolute contraindication) - Relative caution if eGFR 30–60 mL/min/1.73m² - Required to calculate safe lithium dosing and monitoring intervals - Baseline needed to detect lithium-induced CKD progression over time 2. **Electrocardiography (ECG):** - Lithium prolongs the QT interval and can cause T-wave flattening - Baseline ECG allows detection of QT prolongation and comparison with future ECGs - Identifies pre-existing cardiac conduction abnormalities that may contraindicate lithium - Lithium is relatively contraindicated in patients with baseline QT prolongation or arrhythmias ### Standard Baseline Investigations Before Lithium Initiation | Investigation | Rationale | Abnormal Finding = Action | |---|---|---| | Serum creatinine, eGFR | Renal baseline; lithium nephrotoxicity | eGFR <30: contraindicated; 30–60: caution | | ECG | Cardiac baseline; QT prolongation risk | QT >450 ms (men) or >460 ms (women): caution | | TSH, free T4 | Lithium-induced hypothyroidism (25–30% of patients) | Baseline for future comparison | | Serum calcium | Lithium-induced hyperparathyroidism | Hypercalcemia: consider alternative | | Fasting glucose | Metabolic baseline | Hyperglycemia: consider alternative | | Pregnancy test (if female of childbearing age) | Lithium is teratogenic (Ebstein anomaly risk) | Positive: discuss risks vs. benefits | **Mnemonic:** **CREST** — **C**reatinine/eGFR, **R**enal baseline, **E**CG, **S**erum calcium, **T**SH — essential baseline investigations before lithium. ### Monitoring Schedule During Lithium Therapy | Parameter | Baseline | 1 Week | 1 Month | 3 Months | 6 Months | Annually | |---|---|---|---|---|---|---| | Lithium level | — | ✓ | ✓ | ✓ | — | — | | Creatinine/eGFR | ✓ | — | — | ✓ | ✓ | ✓ | | TSH | ✓ | — | — | — | — | ✓ | | ECG | ✓ | — | — | — | — | ✓ (if QT prolongation) | | Serum calcium | ✓ | — | — | — | — | ✓ | **Key Point:** Lithium levels are checked frequently early (therapeutic range 0.6–1.2 mEq/L), but organ function (creatinine, TSH, calcium) is monitored less frequently because lithium-induced organ damage develops over months to years. ### Why This Patient Requires Serum Creatinine, eGFR, and ECG - **Clinical context:** Starting lithium maintenance therapy for confirmed Bipolar Disorder II - **Mandatory baseline:** Renal and cardiac function must be documented before any lithium dose - **Risk stratification:** eGFR determines safe dosing; ECG identifies QT prolongation risk - **Guideline standard:** All major guidelines (APA, NICE, Indian Psychiatric Society) mandate these baseline investigations [cite:Harrison 21e Ch 470] --- ## Why Each Distractor Is Incorrect | Option | Why Wrong | |--------|----------| | 24-hour urine protein and renal ultrasound | These are used to assess chronic lithium-induced CKD *after* it has developed, not as baseline screening. Baseline serum creatinine and eGFR are sufficient to assess pre-existing renal disease. Renal ultrasound is not routine. | | Chest X-ray and echocardiography | Chest X-ray is not indicated for lithium baseline. Echocardiography is not routine; ECG is the appropriate cardiac baseline investigation. Echocardiography is reserved for patients with pre-existing cardiac disease or ECG abnormalities. | | Thyroid peroxidase (TPO) antibodies and anti-thyroglobulin antibodies | While lithium causes hypothyroidism in 25–30% of patients, baseline TSH (not antibodies) is the appropriate screening test. TPO and anti-thyroglobulin antibodies detect autoimmune thyroiditis but do not predict lithium-induced hypothyroidism. | --- ## Summary **Key Point:** Serum creatinine, eGFR, and ECG are the investigation of choice before lithium initiation because they establish baseline renal and cardiac function—the two organ systems most vulnerable to lithium toxicity. These investigations are mandatory to determine safe dosing, identify contraindications, and allow future comparison to detect lithium-induced organ damage.