Diphenhydramine (Antihistamine) Concentration in Plasma
This test measures the amount of diphenhydramine, a common over-the-counter antihistamine, in your blood plasma. It helps assess drug levels, particularly in cases of overdose or therapeutic monitoring.
["This test helps doctors check if your diphenhydramine dose is safe and effective."]
Key facts at a glance
- LOINC
- 73412-9
- Short code
- Diphenox+Atropine ind plt IgM SP Ql FC
- Synonyms
- Diphenox+Atropine ind plt IgM SP Ql FC
- Category
- hematology
- Sample
- serum
- Container
- Red-top tube (SST)
- Units
- N/A
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- Method
- flow-cytometry
- CPT
- 82784
- Specimen volume
- 5 mL
- Stability
- Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
- Hemolysis max
- 500
- Lipemia max
- 1000
- Icterus max
- 500
- TAT
- 4–24 hours
- Order
- routine
Why This Test Is Done
Your healthcare provider may order this test if you’ve taken high doses of diphenhydramine (e.g., Benadryl) or if you’re experiencing unusual symptoms like confusion, slow heart rate, or extreme drowsiness. It can also be used in medical settings to monitor patients receiving this medication for specific conditions.
What to Expect Before the Test
No special preparation is needed. You’ll provide a blood sample drawn from a vein, typically in a lab setting. The sample is processed to measure the drug concentration in your plasma.
What the Results Mean
Results are compared to general reference ranges for diphenhydramine in plasma. High levels may indicate overdose or improper metabolism, while therapeutic levels (if monitored) may confirm appropriate dosing. Your provider will interpret results in the context of your symptoms and medical history.
Common Reasons for Testing
Overdose suspicion: Symptoms like dizziness, irregular heartbeat, or unconsciousness may prompt this test. Monitoring: Patients on long-term diphenhydramine therapy (e.g., for sleep or allergies) may have levels checked to avoid side effects.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
Clinical Significance: Diphenhydramine is a first-generation antihistamine with sedative effects. Elevated levels can impair cognition, coordination, or respiratory function, especially in children or those with liver/kidney issues. Therapeutic monitoring helps prevent toxicity while ensuring efficacy. Always follow your provider’s guidance on medication use.
Short name
Diphenhydramine Level
Frequently asked questions
How long does it take to get results from this test?
What are typical symptoms of diphenhydramine overdose?
Can I drink water before this blood test?
Sources
- 1
- 2Lab Tests OnlineSource
- 3
Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.
This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.
This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.
If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.
Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy