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Ceftriaxone Free Concentration in Serum/Plasma

Published: August 9, 2026Updated: August 15, 2026

This test measures the amount of ceftriaxone (an antibiotic) that is not bound to proteins in your blood, helping doctors assess its effectiveness and potential side effects.

Quick summary

["Understanding your free antibiotic levels can guide treatment decisions and monitor safety."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 15, 2026Editorial policy

Key facts at a glance

LOINC
80957-4
Short code
CEFTRI
Synonyms
cefTRIAXone free SerPl-mCnc
Category
chemistry
Sample
serum
Container
Red-top serum tube (no anticoagulant) or serum separator tube (SST); plasma may be collected in heparin or EDTA tubes
Units
N/A
Ref low
20
Ref high
100
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS) or High-Performance Liquid Chromatography (HPLC)
CPT
80299 (Quantitation of therapeutic drug, not otherwise specified)
SNOMED
395087006
Specimen volume
2-5 mL of whole blood yielding 1-2 mL of serum or plasma
Stability
Serum/plasma stable for 24 hours at room temperature (15-25°C), 7 days at 2-8°C, and 30 days at -20°C or below. Avoid repeated freeze-thaw cycles.
Hemolysis max
100
Lipemia max
300
Icterus max
50
TAT
24–72 hours
Order
routine

Why This Test Is Done

Doctors may order this test if you’re taking ceftriaxone to check how much of the drug is actually available in your bloodstream. Free drug levels are important because they reflect the portion that can actively fight infections, while protein-bound portions may not be as effective.

What to Expect Before the Test

No special preparation is needed. You’ll have a small needle inserted into your arm or a blood sample drawn from a vein. If you’re on blood thinners, let your doctor know before the test.

What the Results Mean

Results help doctors determine if your dose is too high (which could cause side effects) or too low (which might mean the infection isn’t being treated well). Free drug levels are often measured in micrograms per milliliter (mcg/mL).

Common Reasons for Monitoring

This test is particularly useful in patients with kidney problems, those who are heavily ill, or those who may have unusual drug interactions. It can also help adjust doses in children or patients with altered protein levels.

Reference range

Therapeutic range: Not well established for free ceftriaxone; total ceftriaxone therapeutic range approximately 20-100 mcg/mL depending on indication. Free drug levels are typically 5-15% of total concentration due to high protein binding (~85-95%).N/A

Specimen: serum

No special patient preparation required. Sample should be drawn at trough level (just before next dose) for steady-state therapeutic drug monitoring, typically after at least 3-5 doses.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Free ceftriaxone levels are critical for tailoring antibiotic therapy. High free concentrations may indicate toxicity (e.g., seizures, liver issues), while low levels suggest insufficient infection control. This test helps balance efficacy and safety, especially in complex cases like severe infections or renal impairment.

Short name

Free Ceftriaxone Level

Frequently asked questions

Will I feel pain during the blood draw?
You might feel a slight pinch or sting when the needle is inserted, but it usually goes away quickly. Most people describe it as mild discomfort. If you’re anxious, let your healthcare provider know—they may offer a numbing cream or other options.
Can I eat or drink before this test?
You can eat and drink normally unless your doctor specifies otherwise. This test doesn’t require fasting, so no special diet is needed.
How long will it take to get my results?
Results are typically available within 1 to 3 business days, depending on your lab’s processing time. Your doctor will discuss them with you during your follow-up visit.
  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 15, 2026. · Read our editorial policy