Skip to content

Serum or Plasma Concentration of Cephalexin (Antibiotic Level)

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

This test measures the amount of the antibiotic cephalexin in your blood or plasma to check how your body is responding to treatment.

Quick summary

["This helps doctors determine if your dose is effective and adjust it if needed."]

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

Key facts at a glance

LOINC
73458-2
Short code
Cephalexin ind plt IgG SerPl Ql FC
Synonyms
Cephalexin 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’re taking cephalexin (a penicillin-type antibiotic) to treat bacterial infections. It helps ensure the drug is working properly and at the right level in your bloodstream.

What to Expect Before the Test

No special preparation is needed. You’ll have a small needle inserted into your arm or vein to draw blood. If done via plasma, it may be collected from a blood draw or a specialized tube labeled for plasma separation.

What the Test Results Mean

Results are typically reported as the concentration of cephalexin in your blood or plasma. These levels help doctors assess whether your infection is being treated effectively and if your dose should be changed.

Possible Causes of Abnormal Results

Low levels may indicate the antibiotic isn’t working as expected, while very high levels could suggest kidney issues or improper dosing. Always discuss results with your provider.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Monitoring cephalexin levels is critical for infections where the antibiotic must reach specific blood concentrations to kill bacteria effectively. It helps prevent resistance, ensures safety, and tailors treatment to your unique needs.

Short name

Cephalexin Level

Frequently asked questions

What does a ‘positive’ or ‘negative’ result mean for this test?
This test doesn’t yield a qualitative ‘positive/negative’ result for infection—it measures the antibiotic’s concentration. Instead, it provides a quantitative value to guide treatment decisions.
How often should I get this test?
Your provider will determine the timing based on your infection, dose, and response. Typically, it’s done during or shortly after starting treatment, then periodically as needed.
Can I eat or drink before this test?
You can eat and drink normally unless instructed otherwise. Avoid caffeine or alcohol before the draw, as they may affect bloodwork results.
  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