A kidney infection typically announces itself with a combination of urinary symptoms and whole-body signs that a simple bladder infection doesn’t produce. The hallmarks are pain in your back or side, fever with chills, and nausea or vomiting, usually on top of the burning and urgency you’d expect from a urinary tract infection. If you’re experiencing that combination, you likely need treatment promptly, because kidney infections can become serious without antibiotics.
Bladder Infection vs. Kidney Infection
Most kidney infections start as lower urinary tract infections that travel upward. A standard bladder infection causes symptoms concentrated in the lower urinary tract: burning or pain when you pee, frequent and urgent trips to the bathroom, pressure in your lower abdomen, and urine that looks cloudy, bloody, or smells unusual. These symptoms are uncomfortable, but they stay localized.
A kidney infection adds a layer of systemic illness. You feel sick in a way that goes beyond urinary discomfort. The key differences to watch for:
- Pain in your back or side, typically on one side, located below the ribs. This is where the kidneys sit, and the pain can range from a dull ache to sharp tenderness.
- Fever and chills. A bladder infection rarely causes a significant fever. A kidney infection commonly does.
- Nausea or vomiting. Feeling queasy or actually throwing up suggests the infection has moved beyond your bladder.
You can have both sets of symptoms at the same time, since the original bladder infection often hasn’t resolved. But the presence of flank pain plus fever is the clearest signal that your kidneys are involved. Some people describe it as feeling like they have the flu combined with a UTI.
What Causes It
The bacterium E. coli is responsible for roughly 90% of uncomplicated kidney infections. These bacteria normally live in your intestines without causing problems, but when they migrate to the urinary tract and climb from the bladder up through the ureters to the kidneys, they trigger a more serious infection. A small percentage of cases involve other bacteria like Klebsiella, which accounts for about 5% of cases overall.
Anything that makes it easier for bacteria to reach the kidneys raises your risk. This includes a history of frequent UTIs, kidney stones or other blockages that trap urine, a weakened immune system, pregnancy (which compresses the ureters), and use of a urinary catheter. Women get kidney infections more often than men because of shorter urethral anatomy, which gives bacteria a shorter path to travel.
How Doctors Confirm It
Doctors typically diagnose a kidney infection based on your symptoms and a urine test, without needing imaging. If you have flank pain or tenderness, urinary symptoms, and fever, along with a urine sample showing signs of infection, that’s generally enough to start treatment.
The urine test uses a dipstick that changes color when white blood cells are present, a marker called leukocyte esterase. A positive result means your immune system is fighting an infection somewhere in the urinary tract. If the dipstick is positive, the urine is usually examined under a microscope and sent for a culture to identify which specific bacteria are causing the infection and which antibiotics will work against it.
One physical sign doctors check for is tenderness when they tap on your back just below the ribs, at the angle where the lowest rib meets the spine. Sharp pain with this maneuver strongly suggests kidney inflammation.
When Imaging Is Needed
For a straightforward first-time kidney infection, imaging with CT scans or ultrasound isn’t necessary. Most people improve with antibiotics alone. However, if your symptoms haven’t improved after 72 hours of treatment, or if you have complicating factors like a history of kidney stones, a transplanted kidney, or pregnancy, imaging becomes more relevant. For pregnant patients, ultrasound or MRI is preferred over CT to avoid radiation exposure.
Symptoms in Older Adults
Kidney infections can look very different in older adults, which makes them easier to miss. The classic burning sensation during urination may not appear at all, because the immune response changes with age. Instead, the first noticeable signs might be confusion, agitation, increased falls, loss of appetite, or new incontinence in someone who previously had no trouble getting to the bathroom.
This happens because the physical stress of an infection can impair mental function in older adults, particularly those with existing cognitive decline. A kidney infection can make dementia symptoms temporarily worse, which family members sometimes mistake for disease progression rather than an acute, treatable problem. If an older person’s mental state changes suddenly, a urinary infection is one of the first things worth investigating.
Can You Test Yourself at Home?
Over-the-counter UTI test strips can detect signs of a urinary tract infection by checking for white blood cells and nitrites in your urine. They’re reasonably accurate, but they have real limitations. They can miss infections entirely, and a positive result only tells you there’s likely an infection somewhere in your urinary tract. It can’t distinguish between a bladder infection and a kidney infection.
If you have symptoms that suggest kidney involvement (the back pain, fever, nausea pattern), a home test strip isn’t sufficient. You need a proper urine culture to identify the bacteria and guide antibiotic selection. The distinction matters because kidney infections require specific antibiotics and monitoring that a bladder infection may not.
How Kidney Infections Are Treated
Antibiotics are the standard treatment, and most people start feeling better within two to three days. Current guidelines recommend 5 to 7 days of antibiotics for uncomplicated kidney infections, though some types of antibiotics may require a full 7-day course. Complicated cases, including those where bacteria have entered the bloodstream, are typically treated for about 7 days as well, provided you’re improving on the medication.
Many kidney infections can be treated at home with oral antibiotics. You don’t necessarily need to be hospitalized unless you can’t keep fluids or medication down due to vomiting, your symptoms are severe, or there are signs the infection is worsening despite treatment. Drinking plenty of fluids helps your kidneys flush bacteria, and rest supports your recovery during the first few days when symptoms are worst.
Warning Signs That Need Immediate Attention
A kidney infection that spreads to the bloodstream can cause sepsis, a life-threatening emergency. The warning signs include a rapid heart rate, fast breathing, confusion or disorientation, very low blood pressure (which you might notice as dizziness when standing), and producing very little urine. High fever that won’t break, or symptoms that are getting worse rather than better after starting antibiotics, also warrant urgent care.
If you started treatment for what seemed like a bladder infection and you develop back pain, fever, or vomiting in the following days, that’s a sign the infection may have reached your kidneys. Don’t wait for your original prescription to finish. The sooner a kidney infection is treated with the right antibiotic, the lower the risk of complications.