What lab workup is recommended for an 83-year-old female presenting with weakness and dizziness after taking Benadryl (diphenhydramine) for seasonal allergies?

Laboratory Workup for an Elderly Patient with Weakness and Dizziness After Taking Diphenhydramine

For an 83-year-old female presenting with weakness and dizziness after taking Benadryl, a comprehensive laboratory workup should include CBC, comprehensive metabolic panel, cardiac enzymes, urinalysis, and orthostatic vital signs to rule out serious underlying conditions that may be exacerbated by the medication.

Primary Considerations

Diphenhydramine (Benadryl) can cause significant adverse effects in elderly patients, particularly:

• Sedation and cognitive impairment 1
• Orthostatic hypotension 2
• Anticholinergic effects (dry mouth, blurred vision, urinary retention) 2
• Increased risk of delirium symptoms in hospitalized elderly patients 3

Recommended Laboratory Workup

Essential Tests

1. Complete Blood Count (CBC)

   • To evaluate for anemia, infection, or other hematologic abnormalities that could contribute to weakness

2. Comprehensive Metabolic Panel (CMP)

   • Electrolytes (sodium, potassium, chloride, bicarbonate)
   • Renal function (BUN, creatinine) - especially important as elderly patients may have reduced renal clearance of medications
   • Glucose - to rule out hypoglycemia as a cause of weakness/dizziness
   • Liver function tests - to assess hepatic metabolism of diphenhydramine

3. Orthostatic Vital Signs

   • Measure blood pressure and heart rate while lying, sitting, and standing
   • Classical orthostatic hypotension is defined as a sustained decrease in systolic BP ≥20 mmHg or diastolic BP ≥10 mmHg within 3 minutes of standing 2
   • In patients with supine hypertension, a systolic BP drop ≥30 mmHg should be considered significant 2

4. Cardiac Evaluation

   • ECG - to assess for arrhythmias, which can be a cardiac cause of syncope or near-syncope 2
   • Cardiac enzymes (troponin) - especially if chest pain or other cardiac symptoms are present

5. Urinalysis

   • To evaluate for infection or dehydration

Additional Tests to Consider

1. Thyroid Function Tests

   • TSH and free T4 - thyroid dysfunction can present with weakness and is common in elderly patients

2. B12 and Folate Levels

   • Deficiencies can cause weakness and neurological symptoms in elderly patients

3. Medication Level

   • If overdose is suspected, diphenhydramine levels may be obtained (normal therapeutic range: 9-120 ng/mL) 4

Clinical Approach Algorithm

1. Immediate Assessment

   • Stabilize the patient and obtain vital signs including orthostatic measurements
   • Perform focused neurological examination

2. Laboratory Evaluation

   • Order CBC, CMP, ECG, urinalysis as first-line tests
   • Consider cardiac enzymes if cardiac symptoms are present

3. Risk Stratification

   • If orthostatic hypotension is present, consider volume status and medication effects
   • If altered mental status is present, consider diphenhydramine toxicity (especially in elderly) 5

4. Management Considerations

   • IV fluids may be indicated for orthostatic hypotension
   • Consider discontinuation of diphenhydramine and substitution with a less sedating antihistamine if needed for allergy symptoms

Important Caveats

• Diphenhydramine has been associated with a 1.7-fold increased risk of delirium symptoms in hospitalized elderly patients 3
• A dose-response relationship exists between diphenhydramine and adverse cognitive effects in older adults 3
• The five most frequent categories of dizziness in emergency departments are vasovagal syncope/orthostatic hypotension (22.3%), vestibular causes (19.9%), fluid and electrolyte disorders (17.5%), circulatory/pulmonary causes (14.8%), and central vascular causes (6.4%) 6
• Avoid physostigmine if wide-complex tachycardia is present on ECG, as this can worsen sodium channel blockade from diphenhydramine toxicity 4

Remember that elderly patients are particularly susceptible to medication side effects, and diphenhydramine's anticholinergic properties can cause significant adverse effects in this population.