A 52-year-old man with a history of chronic alcohol abuse presents with confusion, ataxia, and ophthalmoplegia. Thiamine (vitamin B1) deficiency is suspected. Which investigation would be most appropriate to confirm thiamine deficiency and assess the severity of neurological involvement?

Explanation

## Confirmatory Investigation for Thiamine Deficiency **Key Point:** Erythrocyte transketolase activity (ETK) with the thiamine pyrophosphate (TPP) effect is the gold standard functional test for thiamine deficiency. It measures both baseline enzyme activity and the percentage increase after TPP addition in vitro. ### Principle of the Erythrocyte Transketolase Test **Mnemonic: ETK-TPP Effect = "Enzyme Activation Ratio"** 1. **Baseline ETK activity:** Measured in lysed red blood cells. 2. **TPP effect:** Percentage increase in enzyme activity after adding exogenous TPP in vitro. - **Normal:** TPP effect <25% (enzyme already saturated with endogenous TPP). - **Thiamine deficiency:** TPP effect >25% (enzyme undersaturated; responds dramatically to added TPP). ### Why ETK Is Superior to Other Tests | Test | What It Measures | Sensitivity | Specificity | Clinical Use | |---|---|---|---|---| | **Serum TPP** | Circulating thiamine pyrophosphate | Low | Low | Unreliable; affected by recent intake | | **Erythrocyte transketolase (ETK) + TPP effect** | Functional enzyme saturation | High | High | **Gold standard** — reflects tissue thiamine status | | **Urine thiamine metabolites** | Renal excretion of thiamine | Moderate | Low | Non-specific; varies with intake and hydration | | **CSF thiamine** | CNS thiamine concentration | High specificity | Limited availability | Invasive; rarely needed; research only | **Clinical Pearl:** In this patient with Wernicke encephalopathy (ophthalmoplegia, ataxia, confusion), the ETK test confirms thiamine deficiency and guides the urgency of supplementation. A TPP effect >25% in the setting of clinical signs is diagnostic. ### Interpretation in Chronic Alcohol Abuse **High-Yield:** Alcoholics have impaired thiamine absorption (intestinal mucosal damage) and reduced hepatic storage. The ETK test will show: - Reduced baseline ETK activity. - Marked TPP effect (>50% in severe deficiency). This indicates that the enzyme is severely undersaturated and urgently needs thiamine replacement. **Warning:** Do not rely on serum thiamine or TPP levels alone — they fluctuate rapidly and do not reflect tissue stores or functional status. The ETK-TPP effect is the only test that reflects the body's true thiamine status.