In short
A culture names the organism; the sensitivity panel lists which antibiotics still work against it. Understanding the S, I and R columns explains why your doctor may change your antibiotic after three days.
Emergency if: Fever with shivering, a racing pulse, confusion, very low urine output or mottled skin — these suggest sepsis and need emergency care now.
On your report
WBC / Total Leukocyte Count, Absolute Neutrophil Count, CRP, ESR +1 more
On this page
01
Two tests printed on one page
A bacteria culture grows a sample — urine, blood, pus, sputum, a wound swab — in the laboratory to determine if you have a bacterial infection and, if so, which type of bacteria is causing it. That is the first half of the report, and it names the organism.
The second half is the antibiotic sensitivity test, which checks how sensitive the bacteria are to different antibiotic medicines. It is only run once something has grown, which is why the sensitivity column is blank on a report that shows no growth.
02
What S, I and R mean
S means susceptible: at the usual dose, that antibiotic is expected to work against this organism at the site of infection. R means resistant: the organism is expected to keep growing despite the drug, and resistant bacteria may continue to grow and multiply, making infections difficult and sometimes impossible to treat.
I sits in between and is the most misread letter on the page. It does not mean the drug half works. In current usage it generally indicates the antibiotic may still succeed where the drug concentrates strongly — in urine, for instance — or at a higher dose, which makes it a conditional yes that depends on where the infection is.
03
Why the report can contradict how you feel
Cultures take time. A urine culture typically reports at one to two days and blood cultures often take longer, so treatment usually starts on an educated guess before the report exists. By the time the sensitivity panel arrives you may already be improving, and if you are genuinely better on a drug the report has marked resistant, your doctor weighs the clinical response rather than switching reflexively.
The reverse also happens: the report shows resistance and you are not improving, which is precisely when the panel earns its keep and the antibiotic changes. This is why finishing the course your doctor settles on, rather than the one you started, is the instruction that matters.
04
Contamination, colonisation and no growth
Not every organism grown is causing disease. A urine sample that grows three different organisms usually reflects contamination during collection rather than an infection with three bacteria, and is repeated with a clean-catch sample. Skin organisms in a single blood culture bottle are often contaminants from the needle passing through skin.
Colonisation is a different idea again: bacteria living on a surface — a catheter, an open wound, the airway — without invading. Bacteria in the urine of someone with no symptoms is usually left alone outside pregnancy and certain procedures, because treating it does not help and drives resistance. A report of no growth after 48 hours often simply means the infection is viral, or that antibiotics were already started before the sample was taken.
05
Getting a usable sample
The quality of the report depends almost entirely on the sample. Wherever possible the sample is taken before the first dose of antibiotic, because even one dose can suppress growth enough to produce a falsely clean result. If you have already started, say so — it changes how the report is read.
For urine, a midstream clean-catch sample sent promptly or refrigerated gives the most reliable answer; a sample left at room temperature for hours lets ordinary skin and genital organisms multiply into numbers that look like infection. For sputum, material coughed from the chest is informative where saliva is not.
When to act
Routine — see a doctor
A mixed-growth urine result with no symptoms: repeat with a clean-catch midstream sample rather than starting an antibiotic.
Same-day — call promptly
Fever, back pain or vomiting with a urine infection, or no improvement after 48 hours on the antibiotic you were given.
Emergency — act now
Fever with shivering, a racing pulse, confusion, very low urine output or mottled skin — these suggest sepsis and need emergency care now.
Values mentioned in this story
Each one opens a visual guide with its normal range and what moves it — upload a report and we place your own numbers on the same scale.
Sources
- Bacteria Culture Test — MedlinePlusmedlineplus.gov
- Antibiotics — NHSnhs.uk
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