What Test Is Done Before Prescribing Antibiotics? Medications and Personalized Treatment

October 14, 2025 •

4 min read

Over 30% of antibiotics are prescribed unnecessarily, contributing to the global crisis of antimicrobial resistance. To combat this, healthcare providers often rely on specific diagnostic procedures, known as antibiotic susceptibility testing, to determine what test is done before prescribing antibiotics. This ensures the medication is effective and appropriate for the infection, preventing the overuse of broad-spectrum drugs.

Quick Summary

Diagnostic testing, including culture and sensitivity tests and rapid assays, is often performed before antibiotic prescriptions to identify the specific pathogen causing an infection and determine its susceptibility to various drugs. Testing helps guide treatment decisions, especially for serious or resistant infections.

Key Points

Identify the specific pathogen: A primary test is a culture and sensitivity (C&S), which identifies the exact microbe causing the infection.
Determine antibiotic effectiveness: The sensitivity part of a C&S test checks how different antibiotics affect the growth of the pathogen, guiding the most effective treatment choice.
Use rapid tests for quick decisions: Point-of-care tests, like the rapid Strep A test, provide fast results to determine if an antibiotic is needed immediately.
Consult antibiograms for empirical therapy: Healthcare providers use antibiograms—reports detailing local susceptibility patterns—to choose the most effective initial antibiotic while waiting for culture results.
Differentiate bacterial from viral infections: New rapid biomarker tests can distinguish between bacterial and viral infections, preventing unnecessary antibiotic prescriptions for viruses.
Prevent antimicrobial resistance: Responsible testing before prescribing helps reduce the overuse of antibiotics, which slows the development and spread of resistant bacteria.

The Importance of Diagnostic Testing in Modern Pharmacology

For decades, antibiotic therapy was often a matter of educated guesswork, or "empirical" treatment, based on a patient's symptoms. However, with the rise of antimicrobial resistance, this practice has become more precise and personalized. Modern medicine places a strong emphasis on diagnostic testing to confirm a bacterial infection and determine the most effective antibiotic, minimizing harm and preserving the efficacy of these life-saving drugs.

What test is done before prescribing antibiotics? The key procedures

When a bacterial infection is suspected, a healthcare provider will typically order one or more tests to confirm the diagnosis and guide treatment. The specific test chosen depends on the site of infection, the severity of the illness, and the need for a rapid versus comprehensive result.

Culture and Sensitivity (C&S) Testing

This is the most common and comprehensive type of testing for many bacterial infections. It involves two main steps:

Culture: A sample of a patient's bodily fluid or tissue—such as blood, urine, sputum, or a swab from a wound—is sent to a laboratory. The sample is then placed in a controlled environment to promote the growth of any microorganisms present. This can take 24 to 48 hours for most bacteria, but longer for some slower-growing organisms.
Sensitivity: Once a sufficient quantity of bacteria has grown, lab technicians test the isolated organism's susceptibility to a panel of different antibiotics. The results show which drugs will be effective (susceptible), which may require a higher dose (intermediate), and which will not work (resistant). This allows a doctor to switch from broad-spectrum (empirical) antibiotics to a more targeted, effective treatment.

Rapid Diagnostic Tests

For certain infections, waiting days for C&S results is not feasible. In these cases, rapid, point-of-care (POC) tests offer quick results to inform an immediate treatment decision.

Rapid Strep Test: Used to diagnose bacterial pharyngitis (strep throat), this swab test provides results in minutes, helping doctors decide if an antibiotic is needed.
Biomarker Tests: A fingerstick blood sample can be used to measure biomarkers like C-reactive protein (CRP) and myxovirus resistance protein A (MxA). CRP levels rise with inflammation, and high levels can suggest a bacterial infection, while MxA is specific to viral infections. The combination of these markers can help distinguish between bacterial and non-bacterial causes, influencing the decision to prescribe an antibiotic.

Antibiograms

Antibiograms are cumulative reports generated by hospitals and laboratories that summarize the susceptibility patterns of local bacterial isolates to various antibiotics over a period of time. While not a test for an individual patient, they are a critical tool for guiding initial, or empirical, antibiotic therapy. Knowing the common resistance rates in a specific facility or region helps a doctor choose an antibiotic that is most likely to be effective before C&S results are available.

Empirical Therapy vs. Targeted Therapy

There are two main approaches to antibiotic treatment, often used in sequence. The diagnostic tests discussed above help transition from a generalized approach to a specific one.


Feature	Empirical Therapy	Targeted (Definitive) Therapy
Timing	Initial treatment, based on symptoms and clinical experience.	After lab results (C&S, etc.) are available.
Scope	Broad-spectrum, designed to cover the most likely pathogens.	Narrow-spectrum, specifically targets the identified pathogen.
Basis	Best guess, informed by guidelines and antibiograms.	Evidence-based, guided by specific lab results.
Primary Goal	Begin treatment quickly to prevent complications, especially in severe infections.	Optimize treatment for effectiveness and minimize side effects and resistance.
Role of Testing	Testing is ordered, but treatment starts before results are in.	Testing is complete, and treatment is adjusted based on the results.

Steps in the diagnostic and treatment process

Clinical Assessment: A doctor evaluates the patient's symptoms, medical history, and severity of illness.
Sample Collection: If a bacterial infection is suspected, a sample is collected (e.g., urine for UTI, throat swab for strep).
Empirical Treatment (if necessary): For severe or time-sensitive infections, an initial broad-spectrum antibiotic may be prescribed based on local antibiogram data and clinical judgment.
Laboratory Analysis: The sample is sent to the lab for culture and sensitivity testing. Rapid tests may be performed for quicker results.
Targeted Therapy: Once C&S results are ready, the doctor adjusts the antibiotic choice and dosage to a more targeted therapy.

Conclusion: The Modern Mandate for Responsible Prescribing

Diagnostic testing before prescribing antibiotics is no longer a best practice; for many infections, it's a critical component of responsible and effective patient care. By confirming the presence of a bacterial infection and identifying the most susceptible pathogen, tests like culture and sensitivity and rapid assays help clinicians transition from broad, empirical therapy to precise, targeted treatment. This approach improves individual patient outcomes, reduces unnecessary antibiotic exposure, and is a vital strategy in the global fight against antimicrobial resistance. The judicious use of these diagnostic tools ensures that antibiotics remain an effective weapon against infection for years to come.

For more information on antibiotic susceptibility testing and antimicrobial resistance, visit the National Institutes of Health (NIH) website.

Frequently Asked Questions

Testing before prescribing antibiotics is important for several reasons. It helps ensure the correct medication is chosen for the specific pathogen, improves the chances of successful treatment, and reduces the risk of ineffective or unnecessary antibiotic use that contributes to global antimicrobial resistance.

A culture and sensitivity (C&S) test is a two-part lab procedure. First, a sample (e.g., blood, urine) is cultured to grow any present microbes. Then, the grown microbes are tested against various antibiotics to see which are effective (sensitive) and which are not (resistant).

Results for bacterial cultures typically take 24 to 48 hours. The process can take longer for some types of bacteria or fungi. This is why doctors may start with an empirical (best-guess) antibiotic before modifying it based on the test results.

A doctor may not perform a test in cases where a bacterial infection is very likely and waiting for results could be dangerous, such as for a life-threatening infection like meningitis. They also may not test for common, uncomplicated infections where the causative bacteria is predictable, like a simple ear infection.

An antibiogram is a report from a hospital or lab that shows the cumulative susceptibility rates of local pathogens to different antibiotics. Clinicians use this data to inform their choice of initial (empirical) antibiotic therapy for common infections in that specific geographic area.

Yes, rapid diagnostic tests exist, particularly for certain infections. Examples include the rapid Strep A test for throat infections and biomarker blood tests (like FebriDx) that can help differentiate between bacterial and viral respiratory infections in minutes.

If a sensitivity test shows that the bacteria causing the infection is resistant to the prescribed antibiotic, the doctor will likely change the medication to a different one that was shown to be effective. In some cases of extensive resistance, a combination of antibiotics might be necessary.