What bloodwork should be checked in a patient presenting with tachycardia, swollen legs, and fatigue?

Laboratory Tests for a Patient with Tachycardia, Swollen Legs, and Fatigue

For a patient presenting with tachycardia, swollen legs, and fatigue, a comprehensive laboratory panel should be ordered including complete blood count, basic metabolic panel, cardiac biomarkers, thyroid function tests, and natriuretic peptides to evaluate for heart failure and other potential causes. 1

Initial Laboratory Evaluation

• Complete blood count (CBC) to assess for anemia, infection, or inflammation that could contribute to tachycardia and fatigue 1, 2
• Serum electrolytes including sodium, potassium, calcium, and magnesium to identify electrolyte abnormalities that may cause or worsen arrhythmias 1
• Renal function tests (BUN, creatinine) with estimated creatinine clearance to assess kidney function and guide medication dosing if heart failure is present 1
• Cardiac biomarkers including troponin to rule out myocardial injury or infarction 1
• B-type natriuretic peptide (BNP) or NT-proBNP to evaluate for heart failure, which is strongly suggested by the clinical presentation 1
• Thyroid function tests (TSH, free T4) to rule out hyperthyroidism as a cause of tachycardia 1
• Blood glucose and HbA1c to screen for diabetes, which can contribute to cardiovascular disease 1
• Liver function tests to assess hepatic congestion from possible heart failure 1

Cardiac-Specific Testing

• Serial troponin measurements if acute coronary syndrome is suspected, particularly with high-sensitivity assays 1
• BNP/NT-proBNP levels are particularly important as they correlate with the severity of heart failure and can help distinguish between cardiac and non-cardiac causes of dyspnea 1

Additional Testing Based on Clinical Suspicion

• D-dimer if pulmonary embolism is suspected as a cause of tachycardia and leg swelling 1
• Arterial blood gas if respiratory compromise is present 1
• Blood cultures if infectious endocarditis is suspected 1

Interpretation Considerations

• Elevated BNP levels support a diagnosis of heart failure but should be interpreted cautiously in older patients, women, and those with renal dysfunction 1
• BNP levels may be lower in obese patients and in heart failure with preserved ejection fraction compared to reduced ejection fraction 1
• Troponin elevation may occur in conditions other than acute myocardial infarction, including heart failure, pulmonary embolism, and renal dysfunction 1
• Hypokalemia is a common adverse effect of diuretic therapy and may cause arrhythmias, while hyperkalemia may complicate therapy with ACE inhibitors, ARBs, and aldosterone antagonists 1

Clinical Pearls

• The combination of tachycardia, leg swelling, and fatigue strongly suggests heart failure, which can be confirmed with elevated BNP/NT-proBNP levels 1, 3
• Persistent tachycardia itself can cause cardiomyopathy (tachycardia-induced cardiomyopathy), leading to heart failure symptoms 3, 4
• Recurrent tachycardia in patients with previous tachycardia-induced cardiomyopathy can cause rapid decline in left ventricular function 4
• Anemia is an important cause of tachycardia and fatigue that should not be overlooked 2
• Thyroid disorders, particularly hyperthyroidism, can present with tachycardia and heart failure symptoms and should always be evaluated 1

By systematically evaluating these laboratory parameters, clinicians can identify the underlying cause of the patient's presentation and guide appropriate therapy.