From the Guidelines
Lethargy is a non-specific symptom that can be caused by various underlying conditions, and the initial laboratory evaluation should include a complete blood count, urinalysis, serum electrolytes, blood urea nitrogen, serum creatinine, glucose, and thyroid-stimulating hormone 1.
Indicated Laboratory Tests
- Complete blood count: to evaluate for anemia, infection, or other hematological disorders
- Urinalysis: to assess for urinary tract infections, kidney disease, or other renal disorders
- Serum electrolytes: to evaluate for electrolyte imbalances, such as hyponatremia or hyperkalemia
- Blood urea nitrogen and serum creatinine: to assess renal function
- Glucose: to evaluate for hyperglycemia or hypoglycemia
- Thyroid-stimulating hormone: to assess for thyroid dysfunction, which can cause lethargy
- Liver function tests: to evaluate for liver disease or dysfunction
- Fasting lipid profile: to assess for hyperlipidemia
Additional Tests
Depending on the clinical presentation and suspected underlying cause of lethargy, additional laboratory tests may be indicated, such as:
- Arterial blood gas analysis: to assess oxygenation and acid-base balance in patients with severe respiratory or cardiac disease 1
- B-type natriuretic peptide (BNP): to evaluate for heart failure, although its use is more established in the diagnosis of acute heart failure 1
- Screening for hemochromatosis or HIV: in selected patients with a high risk of these conditions 1
From the Research
Laboratory Tests for Lethargy
The following laboratory tests are indicated for lethargy:
- Full blood count to check for anaemia, which can cause lethargy 2
- Measurement of iron, vitamin B12, and folate levels to diagnose nutritional deficiencies that can lead to anaemia and lethargy 2
- Basic metabolic panel to evaluate electrolyte levels, blood urea nitrogen, and creatinine, which can help identify underlying renal or metabolic disorders 3
- Clinical laboratory tests such as circulating immune complexes, atypical lymphocytosis, immunoglobulin G, alkaline phosphatase, total cholesterol, and lactic dehydrogenase to investigate chronic fatigue syndrome, which can cause lethargy 4
Limitations of Laboratory Tests
It is essential to note that:
- Abnormal laboratory results do not necessarily indicate the cause of fatigue or lethargy 5
- Laboratory tests may not be useful in guiding evaluation or treatment for fatigue or lethargy in some cases 5
- Each laboratory finding lacks sufficient sensitivity to be a diagnostic test for chronic fatigue syndrome or lethargy 4