Kidney Infection Symptoms: Early Signs and When to Go to the ER

Burning when you urinate is annoying. Burning plus fever and back pain is a warning. Kidney infection symptoms like these often mean a urinary tract infection (UTI) has climbed from the bladder to the kidneys, where it can turn serious fast.

Alerna Kidney Health shares plain-language kidney guides like this one, so bookmark it or send it to someone who gets frequent UTIs. Knowing the early signs, how they differ from look-alike problems, and when to head to the emergency room (ER) can help you get treated before the infection causes lasting damage.

A kidney infection, also called pyelonephritis, is a bacterial infection of the upper urinary tract. It rarely starts in the kidney itself. Your two kidneys filter waste from your blood to make urine, and bacteria usually reach them by moving up from lower in the urinary system.

A kidney infection is a type of urinary tract infection, just a more serious one. Most begin when bacteria enter the urethra, the tube that carries urine out of the body, and infect the bladder. The most common cause is Escherichia coli (E. coli), a type of bacteria that normally lives in the gut. If the bladder infection isn't treated, the bacteria can climb the ureters, the tubes that connect the bladder to the kidneys.

Women face a higher risk because they have a shorter urethra, so bacteria have less distance to travel. Other risk factors include sexual activity, pregnancy, and using a urinary catheter. Kidney stones or an enlarged prostate gland can raise the risk by blocking urine flow, while diabetes and any health condition that causes a weakened immune system make infections harder to fight.

A kidney infection may affect one or both kidneys, though it often stays on one side. That's why the pain tends to show up on one side of the back, just below the ribs. Pain in that spot, paired with fever or urinary symptoms, helps set a kidney problem apart from ordinary back pain.

The early signs of a kidney infection often look like a bladder infection that suddenly gets worse. Fever and pain in your back or side are the clues that the infection has likely moved higher.

Kidney infection symptoms often come on fast and may include:

• Fever and chills

• Pain in the back, side (flank), or groin

• Pain in the lower abdomen

• A burning sensation or pain when you urinate

• Frequent urination or a strong urge to go, even when little comes out

• Cloudy, dark, or foul-smelling urine

• Blood in the urine, which may look pink or red

• Nausea or vomiting

• Feeling very tired or unwell

Older adults may not have the usual symptoms of a kidney infection and may seem confused instead. Young children may have only a fever or seem unusually fussy.

A bladder infection, kidney stones, and a kidney infection share some symptoms, so they are easy to mix up. Fever with back pain usually points to the kidneys. The type of pain helps sort out the rest.

A bladder infection often causes a burning feeling when you pee, a frequent urge to go, and pressure low in the pelvis. It usually doesn't cause a high fever or back pain, although a mild fever can happen. Waiting it out is risky, since an untreated bladder infection can travel up to the kidneys.

Kidney stones often cause sudden, severe pain in the side or back that comes and goes in waves and may spread to the groin. Urine can look pink, red, or brown. A stone by itself usually doesn't cause a fever, so stone pain with fever or chills needs emergency care. That mix can mean a blocked kidney is also infected, which is dangerous.

Emergency care is needed when kidney infection symptoms come with signs of a severe kidney infection or a reaction that affects your whole body. These warning signs can get worse within hours, so don't wait for a regular appointment.

Go to the ER or call 911 if you notice any of these signs:

• High fever with shaking chills

• Vomiting that keeps fluids or medicine from staying down

• Confusion, extreme sleepiness, or trouble staying awake

• A fast heartbeat or fast breathing

• Feeling faint or dizzy when you stand

• Little or no urine

• Cold, clammy, or blotchy skin

• Kidney stone pain with fever

• Any kidney infection symptoms during pregnancy

These can be signs of sepsis, a life-threatening reaction that can develop when the infection spreads to the blood. Milder symptoms, like fever and back pain without these red flags, still mean you should seek medical care the same day from a doctor or urgent care clinic.

A kidney infection needs prescription antibiotics, and where you get them depends on how sick you are. Early treatment usually means a faster recovery. Left untreated, the infection can lead to serious complications, including permanent kidney damage and sepsis.

A healthcare provider will usually order urine tests to confirm the infection and find out which bacteria are behind it. Those results help guide the choice of antibiotics. Many people feel better within a few days, but finishing every dose helps prevent complications and lowers the chance the infection comes back.

While the medicine works, drink plenty of water to stay hydrated unless your doctor has told you to limit fluids. Ask which pain reliever is safe for your kidneys, since some common options can be hard on them.

Severe cases often need a hospital stay. Pregnant women, people who can't keep pills down, and anyone with ER warning signs may get intravenous (IV) antibiotics and fluids, which go directly into a vein. Most people switch to pills once they start to improve.

Your doctor may order a repeat urine test after treatment. Imaging tests, such as an ultrasound or a computed tomography (CT) scan, may be needed if infections keep coming back or a stone may be blocking urine flow. Repeated infections can scar the kidneys, which raises the risk of kidney disease and high blood pressure later on.

If urinary infections keep returning, ask a healthcare professional which habits may help lower your risk. Common advice includes drinking more water, peeing soon after sex, not trying to hold urine for long stretches, wiping from front to back, and skipping douches or sprays in the genital area.

Fever, chills, and pain in your back or side on top of burning or frequent urination are the clearest early signs of a kidney infection, and catching them early protects your kidney health. Confusion, a racing heartbeat, little or no urine, or kidney stone pain with fever are red flags that call for emergency care.

Once treatment is done, Alerna’s D-Mannose, made with D-mannose, cranberry, and vitamin C, can fit into an everyday urinary wellness routine, but it can’t treat a kidney infection. Call your doctor today if these symptoms sound familiar, and if you see any ER warning signs, go now.

The information provided in this article is for educational and informational purposes only and should not be considered a substitute for professional medical advice, diagnosis, or treatment. Please consult with your healthcare provider before starting any new dietary supplement, especially if you are pregnant, nursing, have a medical condition, or are taking other medications. Never disregard professional medical advice or delay in seeking it because of something you have read in this article.

1. Belyayeva, M., Leslie, S. W., & Rout, P. (2024). Acute pyelonephritis. In StatPearls. StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK519537/

2. Burrows, L. L. (2024). It's uncomplicated: Prevention of urinary tract infections in an era of increasing antibiotic resistance. PLOS Pathogens, 20(2), e1011930. https://doi.org/10.1371/journal.ppat.1011930