Blood Test Guide

Blood Culture Test

Name: Blood Culture Test: What It Is & When It's Done
Creator: Suman Konda
Published: 2026-07-05

A blood culture test detects bacteria or fungi in the bloodstream: it's used to diagnose sepsis and other serious blood infections. Here's what you need to know.

Written and clinically reviewed by Suman Konda, PharmD, Clinical Pharmacist · Based on peer-reviewed sources · Editorial policy · Not medical advice

Last reviewed and updated: 01 August 2026 · How we check our content

When is a blood culture ordered?

Situation	Why blood culture is needed
Suspected sepsis	Bacteria in the blood require urgent antibiotics
High fever with no obvious source	Identify if bacteria are circulating
Suspected endocarditis	Heart valve infection: multiple cultures needed
Line infection (IV catheter)	Catheters can introduce bacteria into the bloodstream
Meningitis workup	Blood culture alongside CSF culture
Immunocompromised patients	Higher risk of unusual organisms

How blood cultures work

The collection and incubation process

Blood is collected under strict sterile technique into two special bottles: one for aerobic organisms (need oxygen) and one for anaerobic organisms (grow without oxygen). The bottles are placed in an automated incubator that monitors for bacterial growth up to 5 days. If growth is detected, the organism is identified and antibiotic sensitivity testing (susceptibility testing) is performed. Results are reported in 24–72 hours if positive, or after 5 days if negative.

Interpreting blood culture results

Result	Meaning
Positive: true bacteraemia	Pathogenic bacteria confirmed: requires antibiotics
Positive: contaminant	Skin bacteria (eg. coagulase-negative Staph): usually from collection, not infection
Negative after 5 days	No bacterial or fungal growth detected

What is sepsis?

Sepsis is a life-threatening organ dysfunction caused by the body's dysregulated response to infection. Bacteria in the blood (bacteraemia) trigger a massive immune response that can damage the heart, kidneys, lungs and brain. Signs include high fever or low temperature, rapid heart rate, low blood pressure, confusion and reduced urine output. Blood cultures are collected before antibiotics are started: but antibiotics must not be delayed waiting for results.

Questions to ask your doctor

What organism grew in my blood culture?
Is the result a true infection or a contaminant?
Which antibiotics is the organism sensitive to?
Do I need further imaging to find the source of infection?

Frequently Asked Questions

What is a blood culture used for?

It checks for bacteria or fungi in the bloodstream, helping diagnose serious infections such as sepsis and identify the exact organism so the right antibiotic can be chosen.

Why are two samples often taken?

Taking cultures from different sites (and before antibiotics) improves accuracy and helps distinguish a true bloodstream infection from skin contamination of the sample.

How long do blood culture results take?

Preliminary results can appear within 24–48 hours, but final identification and antibiotic sensitivity often take up to 5 days, so initial treatment is usually started before results return.

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

Febrile Neutropenia. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK541102