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Serum/plasma concentration of diphenhydramine (antihistamine)

Published: August 9, 2026Updated: August 16, 2026

This test measures the amount of diphenhydramine, a common antihistamine, in your blood. It helps assess drug levels, especially if you’ve taken this medication for allergies or sleep.

Quick summary

["Check your antihistamine levels to ensure safe and effective dosing."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
73415-2
Short code
Diphenhy ind plt IgG SerPl Ql FC
Synonyms
Diphenhy ind plt IgG SerPl 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 been prescribed diphenhydramine (e.g., Benadryl) and want to confirm your blood levels. It’s particularly useful for monitoring in children, elderly patients, or those taking high doses. Rarely, it may be checked if you experience unusual side effects like dizziness or confusion.

What to Expect Before the Test

No special preparation is needed. You’ll provide a blood sample, usually drawn from a vein in your arm. If you’re taking diphenhydramine regularly, your provider may ask you to stop taking it for a few days before testing to get an accurate reading.

How the Test Works

Blood is collected and analyzed using an immunoassay method to detect diphenhydramine. This test measures the substance’s concentration in your serum or plasma.

Possible Results and Interpretation

Results are reported in micrograms per milliliter (mcg/mL). While reference ranges vary by lab, high levels may indicate overdose or improper dosing, while low levels could suggest insufficient medication. Your provider will interpret results based on your medical history and symptoms.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance

Short name

Diphenhydramine Level

Frequently asked questions

What does my diphenhydramine level mean?
High levels might suggest you’ve taken too much, which can cause drowsiness, confusion, or slowed breathing. Low levels could mean your medication isn’t working as expected. Always discuss results with your doctor, as they’ll consider your full medical picture.
How often should I get this test?
Typically, this test isn’t routine unless you’re on long-term high-dose diphenhydramine or have specific concerns about your levels. Your provider will decide if monitoring is needed based on your situation.
Can I eat or drink before this test?
You can eat and drink normally before the test, as it doesn’t require fasting. However, if you’re taking diphenhydramine, ask your provider if stopping it temporarily is advised for accuracy.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

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.

Medical Disclaimer

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