A kidney infection typically announces itself with fever, chills, and pain in your lower back or side, usually on top of the burning and frequent urination you’d expect from a bladder infection. If you already have UTI symptoms and suddenly feel much worse, with a fever above 101°F and deep pain near your ribs, that combination strongly suggests the infection has moved beyond your bladder and into one or both kidneys.
How a Kidney Infection Feels Different From a Bladder Infection
Bladder infections and kidney infections share some symptoms: painful urination, the constant urge to go, and frequent trips to the bathroom. The overlap makes sense because a kidney infection almost always starts as a bladder infection. Bacteria from your intestinal tract colonize the area around your urethra, travel up into the bladder, and in some cases keep climbing through the tubes that connect the bladder to the kidneys (the ureters) until they reach the kidney itself.
The key difference is systemic illness. A bladder infection is uncomfortable but localized. You feel it when you pee. A kidney infection makes your whole body respond. The hallmark signs that the infection has reached your kidneys include:
- Fever and chills: A bladder infection rarely causes a fever. If your temperature spikes alongside urinary symptoms, the infection has likely spread.
- Flank or lower back pain: This pain sits deep in your side, roughly where your lowest ribs meet your spine. It often affects only one side and feels different from a pulled muscle. It can be sharp or a deep, constant ache.
- Nausea and vomiting: The infection triggers an inflammatory response strong enough to upset your stomach.
Symptoms often begin suddenly. You might feel fine in the morning and be shivering with back pain by evening. Some people notice their urine looks cloudy, dark, or has a strong smell, though these signs alone are not specific to kidney infections.
Where Exactly the Pain Shows Up
The pain from a kidney infection concentrates in what doctors call the costovertebral angle, the spot where the bottom of your rib cage meets your spine on either side. If you put your hand on your lower back, just below and to the side of your last rib, that’s roughly where an inflamed kidney sits. The pain is usually on one side because the infection typically affects one kidney at a time.
Doctors check for this by firmly tapping that area during a physical exam. If it produces a sharp, wincing pain, it’s a reliable indicator of kidney inflammation. This tenderness is highly specific to kidney problems, meaning if it hurts there, the kidneys are very likely involved. That said, the test isn’t perfectly sensitive: some people with a kidney infection won’t have dramatic tenderness on exam, especially early on.
Symptoms That Look Different in Certain People
Kidney infections don’t always follow the textbook pattern. In older adults, people with nerve damage affecting the bladder, or those with long-term urinary catheters, a kidney infection may not produce obvious urinary symptoms at all. Instead, it can show up as sudden confusion, a drop in mental sharpness, or a general decline that’s easy to mistake for something else entirely. This is one reason kidney infections in older adults can become dangerous before anyone recognizes the cause.
Pregnancy raises the stakes significantly. Kidney infections complicate up to 2% of pregnancies in the United States, making them one of the most common reasons for hospitalization during pregnancy outside of delivery-related issues. Pregnant women with untreated bacteria in their urine (even without symptoms) have a 20 to 35% chance of developing a kidney infection later in pregnancy. Screening and treating that silent bacteria early drops the risk to 1 to 4%. When kidney infections do occur during pregnancy, they carry a much higher risk of serious complications, including bloodstream infection (56 times the risk compared to pregnancies without a kidney infection) and preterm birth.
Diabetes, kidney stones, and structural abnormalities in the urinary tract also increase your risk. These conditions either slow the flow of urine (giving bacteria more time to multiply) or weaken the immune response that would normally keep infections contained to the bladder.
What Testing Actually Involves
If you suspect a kidney infection, a healthcare provider will typically start with a urinalysis. This test checks your urine for white blood cells (a sign your immune system is fighting something) and bacteria. One particularly telling finding is the presence of white blood cell casts, clumps of white blood cells that form in the tiny tubes inside the kidney. These casts are specific to kidney-level infection and help distinguish it from a simple bladder infection.
A urine culture, where a lab grows bacteria from your sample, confirms exactly which organism is causing the infection and which antibiotics will work against it. Cultures are far more reliable than dipstick tests alone.
Over-the-counter UTI test strips, the kind you can buy at a pharmacy, check for two things: an enzyme released by white blood cells and nitrites produced by certain bacteria. These strips catch 80 to 92% of UTIs and are fairly good at confirming bacteria when present (96 to 99% accuracy for nitrites). But they have real limitations. A positive result tells you there’s likely an infection somewhere in your urinary tract. It cannot tell you whether that infection is in your bladder or your kidneys. And a negative result doesn’t guarantee you’re infection-free. Home strips can give you a starting signal, but they’re no substitute for a proper evaluation when kidney infection symptoms are present.
When Symptoms Become an Emergency
Most kidney infections are treatable with oral antibiotics and don’t require hospitalization. But a kidney infection that goes untreated or doesn’t respond to treatment can progress to urosepsis, a condition where the infection enters the bloodstream and triggers a dangerous whole-body inflammatory response.
Get to an emergency room if you notice any of the following alongside your symptoms:
- Fast heart rate or heart palpitations
- Rapid breathing (faster than about 22 breaths per minute)
- Difficulty breathing
- Feeling lightheaded or faint, which can signal dropping blood pressure
- Inability to urinate at all
- A weak pulse
High fever that won’t come down, persistent vomiting that prevents you from keeping fluids or medication down, or worsening pain despite treatment are also reasons to seek urgent care.
What Recovery Looks Like
Once you start antibiotics for a kidney infection, symptoms often begin improving within a few days. The fever usually breaks first, followed by gradual relief of the back pain and nausea. Even though you’ll feel noticeably better early on, the full course of antibiotics typically lasts a week or longer. Finishing the entire course matters because stopping early gives surviving bacteria a chance to regrow and potentially develop resistance.
During recovery, staying hydrated helps your kidneys flush bacteria more effectively. Most people with uncomplicated kidney infections recover fully at home. If your symptoms haven’t improved after 48 to 72 hours of antibiotics, your provider may order additional testing or adjust your medication, since some bacteria are resistant to common first-line treatments. Severe cases, particularly those involving high fevers, an inability to keep oral medication down, or signs of sepsis, require intravenous antibiotics in a hospital setting.