UTI vs Kidney Infection: Symptoms, Key Differences and When It's Urgent

Written by Cherlie Joseph, MSN, FNP-BC · Medically reviewed by Naurah Gaspard, DNP, FNP-C, FNP-BC · Updated November 2026

A typical UTI is a bladder infection: burning when you pee, urgency, peeing often and pressure low in the belly. A kidney infection (pyelonephritis) usually adds fever, chills, pain in your back or side, and nausea or vomiting. A kidney infection is more serious, needs prompt treatment with the right antibiotic, and often needs an in-person exam.

UTI vs kidney infection at a glance

"UTI" covers any infection in the urinary tract. Most are in the bladder (cystitis), the lower part of the tract. A kidney infection is an upper urinary tract infection, and most start as a bladder infection that travels up the ureters.

	Bladder infection (cystitis)	Kidney infection (pyelonephritis)
Main symptoms	Burning, urgency, frequency, lower belly pressure, sometimes blood in urine	Bladder symptoms plus fever, chills, back or side pain, nausea or vomiting
Fever	Usually none	Common
Feels like	Uncomfortable but you can function	Often sick, achy and wiped out
Tests	Urinalysis, culture if recurrent or unclear	Urine culture, often blood work; imaging in some cases
Usual care setting	Often treated online or in primary care	In-person exam; sometimes IV antibiotics or hospital care

Symptoms that point to the kidneys

According to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), kidney infection symptoms include:

• Fever and chills
• Pain in the back, side (flank) or groin
• Nausea or vomiting
• Frequent or painful urination
• Cloudy, dark, bloody or foul-smelling urine

Kidney pain is usually felt in the side or back just under the ribs, often on one side, and it is deeper and steadier than a pulled muscle. Low back pain that changes with movement and comes without fever or urinary symptoms is more likely to be muscular. Older adults may have fewer classic signs and instead seem confused or unusually weak.

How a bladder infection spreads, and who is at higher risk

Most kidney infections begin when bladder bacteria, usually from the bowel, move upward. Less often, bacteria reach the kidneys through the blood. A bladder infection that is left untreated, or treated with an antibiotic the bacteria resist, has more time to spread.

Your risk of a kidney infection, or of a UTI becoming more serious, is higher if you:

• Are pregnant
• Have diabetes or a weakened immune system
• Have kidney stones or another blockage in the urinary tract
• Have an enlarged prostate or trouble emptying your bladder
• Use a urinary catheter
• Have had a kidney infection before

How treatment differs

Bladder infections are usually treated with a short course of an antibiotic such as nitrofurantoin, fosfomycin or trimethoprim-sulfamethoxazole, chosen based on your allergies, history and local resistance patterns.

Kidney infections need an antibiotic that reaches good levels in kidney tissue and is typically taken for longer. Nitrofurantoin and fosfomycin concentrate in urine but are generally not used for kidney infections for this reason. A urine culture is important so the antibiotic can be adjusted if the bacteria are resistant. People who are vomiting, very ill, pregnant or not improving may need IV antibiotics or hospital care. Untreated kidney infections can lead to kidney damage or sepsis, a life-threatening reaction to infection.

Whichever infection you have, you should feel noticeably better within a couple of days of starting the right antibiotic. If you don't, or you get worse, contact a provider promptly.

Can you treat it online?

Many simple bladder infections in adults can be safely treated through an online visit. At OurTopClinic, a Florida-licensed nurse practitioner reviews your symptoms and history, can order a urine test at a nearby lab, and can send a prescription to your pharmacy if treatment is appropriate. A prescription is never guaranteed.

If your answers suggest a kidney infection (fever, chills, flank pain, vomiting), we will usually direct you to in-person care, because you may need an exam, blood work, a culture before starting antibiotics and sometimes IV treatment. That is the safest path, not a brush-off.

Frequently asked questions

How do I know if my UTI has spread to my kidneys?

New fever, chills, pain in your back or side, or nausea and vomiting on top of bladder symptoms are the main warning signs. If these appear, get seen in person the same day.

Can a kidney infection go away on its own?

You should not wait it out. A kidney infection needs antibiotics, and delays raise the risk of kidney damage and sepsis.

Can you have a kidney infection without a fever?

It is possible, especially in older adults or people with weakened immune systems, but fever is common. Flank pain with urinary symptoms deserves an exam even without fever.

Sources

• NIDDK: Symptoms and Causes of Kidney Infection (Pyelonephritis)
• CDC: Urinary Tract Infection Basics
• CDC: Clinical Overview of Urinary Tract Infections

Keep reading

• UTI treatment online in Florida
• Recurrent UTI causes and prevention
• UTIs after menopause
• Browse all conditions and treatments A to Z

This article is for general education and is not a substitute for medical advice. Treatment, including any prescription, is decided by a licensed provider after reviewing your health history. OurTopClinic is not for emergencies; if you have a medical emergency, call 911. OurTopClinic serves adults located in Florida, New Mexico, Oregon and Washington. Learn more at ourtopclinic.com.