What blood tests should be arranged for an elderly patient with suspected delirium?

Blood Tests for Suspected Delirium in Elderly Patients

Order a comprehensive metabolic panel, complete blood count with differential, TSH, vitamin B12, and urinalysis immediately, along with point-of-care glucose and pulse oximetry—these identify the vast majority of reversible causes of delirium. 1

Immediate Point-of-Care Testing (First Priority)

• Check finger-stick blood glucose immediately upon patient contact, as hypoglycemia is rapidly reversible and can be fatal if missed, with signs frequently mimicking other conditions 1
• Obtain pulse oximetry to assess for hypoxia, which is a common precipitant of confusion 1
• If glucose is low, administer 30-50 g IV glucose urgently 1

Core Laboratory Panel (Order in All Cases)

The following tests identify the most common reversible causes of delirium and should be obtained in virtually all elderly patients with suspected delirium 1:

Complete Metabolic Panel

• Electrolytes (sodium, potassium, chloride, bicarbonate) to identify hyponatremia and hypernatremia, among the most common reversible metabolic causes 1
• Renal function (BUN, creatinine) to assess for uremia and dehydration 1
• Hepatic panel (AST, ALT, bilirubin, albumin) to identify hepatic encephalopathy 1
• Glucose to confirm normoglycemia 1
• Calcium to detect hypercalcemia, which is reversible in 40% of cases and can present with subtle confusion 2
• Magnesium and phosphate to identify additional electrolyte disturbances 1

Complete Blood Count with Differential

• Identifies infection, anemia, and hematologic abnormalities that can precipitate or worsen confusion 1
• White blood cell count and differential help detect occult infection, even when bacteremia may not show obvious laboratory abnormalities initially 2

Thyroid Function

• TSH level is mandatory, as thyroid dysfunction is a reversible cause of confusion and can present with new psychiatric symptoms in the elderly 1

Vitamin B12 and Homocysteine

• Vitamin B12 deficiency causes neuropsychiatric symptoms and cognitive decompensation 1
• Homocysteine should be included to indicate functional B12 deficiency 1

Urinalysis

• Obtain routinely as part of the initial evaluation to identify urinary tract infection, the most common infectious precipitant of confusion in elderly patients 1
• Critical caveat: Treat urinary tract infection only when the patient meets systemic sepsis criteria, as treating asymptomatic bacteriuria worsens functional recovery and increases Clostridium difficile infections 2

Context-Specific Additional Testing

When Infection is Suspected

• Chest X-ray if pulmonary infection is suspected 1
• Blood cultures if sepsis is a concern 1
• Consider urine culture if urinalysis is abnormal and patient meets sepsis criteria 1

When Cardiac Etiology is Suspected

• 12-lead ECG to evaluate for arrhythmias or myocardial ischemia 1
• Cardiac troponin if myocardial ischemia is suspected 1

When Substance Use is Suspected

• Toxicology screen and blood alcohol level 1
• Medication levels for drugs with narrow therapeutic windows 1

When Respiratory Concerns are Present

• Arterial blood gas analysis, as hypoxia from pulmonary embolism can present as acute confusion 1

Specialized Testing (Second Tier)

The following tests should be considered when the etiology remains uncertain after initial workup or in atypical cases 3:

• Methylmalonic acid (MMA) for functional B12 deficiency 3
• Ammonia if hepatic encephalopathy is suspected 3
• HgbA1c to assess glycemic control 3
• Folate for nutritional deficiency 3
• Lipid profile if vascular etiology is considered 3

Critical Pitfalls to Avoid

• Never assume intoxication without first ruling out hypoglycemia, as clinical presentations overlap significantly and hypoglycemia can cause permanent brain damage 1
• Do not overlook dehydration, which is a common precipitating factor that may not be evident on initial laboratory studies 2
• Avoid empirical treatment of asymptomatic bacteriuria, as it results in worse functional recovery and higher C. difficile infection rates 2
• Review all medications for anticholinergic burden (including antihistamines like cyclizine), benzodiazepines, and opioids, as these are strong precipitants of delirium 2

Underlying Causes to Systematically Evaluate

Beyond laboratory testing, the comprehensive evaluation should address 3:

• Infections: Urinary tract infection and pneumonia are most common 3
• Medications: Anticholinergic medications, benzodiazepines, and opioids (particularly in renal impairment) 3, 2
• Dehydration and electrolyte disturbances 3
• Pain: Unrecognized pain is systematically undertreated in elderly patients with cognitive impairment 2
• Hypoxia: Maximize oxygen delivery with supplemental oxygen, blood, and blood pressure support as needed 3
• Urinary retention and constipation 3