A kidney infection typically announces itself with a combination of flank pain, fever above 100.4°F (38°C), and nausea or vomiting. These symptoms set it apart from a regular bladder infection, which usually stays limited to burning with urination and frequent bathroom trips. If you’re experiencing back or side pain along with feeling sick all over, there’s a good chance the infection has moved beyond your bladder.
How Kidney Infections Differ From Bladder Infections
Bladder infections (cystitis) and kidney infections (pyelonephritis) are both urinary tract infections, but they affect different parts of the system and feel very different. A bladder infection tends to cause local, lower-body symptoms: burning when you pee, feeling like you need to go constantly, pressure above your pubic bone, and sometimes visible blood in your urine. You generally don’t feel sick overall.
A kidney infection, by contrast, makes your whole body react. Fever above 100.4°F is one of the clearest dividing lines. Bladder infections either cause no fever at all or only a mild, low-grade one. Once the infection reaches your kidneys, your immune system mounts a much stronger response, which is why you’ll often feel chills, nausea, vomiting, and general exhaustion on top of any urinary symptoms. Some people with kidney infections don’t have obvious bladder symptoms at all, which can make it confusing.
The Signature Pain Pattern
The most recognizable sign of a kidney infection is pain in your flank, the area on your back between your lower ribs and your hip. It’s usually on one side, since kidney infections most often affect a single kidney. The pain can range from a dull, persistent ache to a sharp tenderness that worsens when you move or when someone presses on the area.
Doctors check for this by tapping on the back at the angle where the lowest rib meets the spine, called the costovertebral angle. If that tap causes a jolt of pain, it’s a strong clinical indicator that the kidney is inflamed. You can roughly test this yourself: if pressing firmly or gently thumping that area on your back makes the pain significantly worse, that’s meaningful. This pain location is distinct from the lower abdominal pressure of a bladder infection, which sits much further down and closer to the front of your body.
How Quickly Symptoms Develop
Kidney infection symptoms typically develop within several hours to about a day. In many cases, people notice bladder infection symptoms first (burning, urgency, frequent urination) and then, within a day or two, start feeling feverish with new back pain. That progression from “annoying but manageable” to “I feel genuinely ill” is a key signal that the infection has traveled upward from the bladder through the tubes that connect to the kidneys.
Not everyone follows this pattern, though. Some people develop kidney infection symptoms without ever noticing a bladder infection stage, particularly if the lower urinary symptoms were mild enough to overlook.
Symptoms That Look Different in Older Adults and Children
Kidney infections don’t always present the textbook way, especially at the extremes of age. In older adults, particularly those living in care facilities, the classic symptoms of flank pain and fever may be absent or muted. Instead, the most common sign is a change in mental status: new confusion, increased agitation, a noticeable decline in how well someone functions day to day, or loss of appetite. In one study of nursing home residents with advanced dementia, mental status changes accounted for over 40% of suspected urinary tract infections, while localized urinary symptoms were uncommon.
Children under 2 may show only a high fever with no other obvious symptoms. They can’t describe flank pain or urinary burning, so an unexplained fever in a young child often prompts a urine test to rule out a kidney infection.
What Happens at the Doctor’s Office
Diagnosing a kidney infection starts with a urine sample. A urinalysis checks for white blood cells (a sign your body is actively fighting infection) and blood in the urine. If bacteria are present, a urine culture identifies the specific type and which antibiotics will work against it.
Over-the-counter UTI test strips can detect some signs of infection at home, particularly nitrites in the urine, which are produced when certain bacteria break down chemicals in your pee. Nitrite testing is quite specific, meaning a positive result reliably indicates bacteria are present. However, these strips tell you that you have a urinary tract infection somewhere. They can’t distinguish between a bladder infection and a kidney infection. That distinction requires the full clinical picture: your symptoms, a physical exam, and sometimes imaging.
Imaging like an ultrasound or CT scan isn’t routine for a straightforward kidney infection. Doctors order it when symptoms don’t improve with antibiotics, when they suspect a blockage like a kidney stone trapping infected urine, or when the infection is unusually severe. An ultrasound can reveal swelling in the kidney, while a CT scan provides a more detailed look at obstructions and complications.
Warning Signs That Need Immediate Attention
Most kidney infections are treatable with oral antibiotics and resolve without lasting damage. But in some cases, the infection can spread into the bloodstream, triggering a dangerous body-wide response called sepsis. This is a medical emergency.
The signs that a kidney infection may be progressing to something more serious include a rapid heart rate, fast breathing, confusion or difficulty staying alert, very high or very low body temperature, low blood pressure (which can feel like lightheadedness or near-fainting when standing), shaking chills, and skin that feels clammy or unusually warm. Producing very little urine is another red flag, since it can signal that the kidneys are struggling. If you or someone you’re with develops several of these symptoms together, that’s a situation for the emergency room, not a wait-and-see approach.
Why Kidney Infections Shouldn’t Be Ignored
Unlike bladder infections, which are uncomfortable but rarely cause lasting harm, kidney infections carry a risk of permanent kidney scarring if they’re severe or go untreated. This is especially true for young children, whose developing kidneys are more vulnerable. In adults, a single properly treated kidney infection is unlikely to cause long-term problems, but repeated infections or delayed treatment increases the risk of damage over time.
The practical takeaway: if your symptoms have moved beyond simple burning and urgency to include fever, flank pain, or vomiting, you’re dealing with something that needs professional treatment rather than home remedies or over-the-counter pain relief alone. Antibiotics for kidney infections are typically prescribed for a longer course than those for bladder infections, and your doctor may want a follow-up urine test to confirm the infection has fully cleared.