The two hallmark signs of a kidney infection are pain beneath your lower ribs on one side of your back and a fever. If you have both of those symptoms, especially alongside urinary symptoms like burning or frequent urination, a kidney infection is likely and you need medical attention quickly. A bladder infection (lower UTI) typically stays limited to discomfort during urination, but once the infection travels up to one or both kidneys, the symptoms shift noticeably and become harder to ignore.
Key Symptoms That Point to a Kidney Infection
A kidney infection, medically called pyelonephritis, produces a distinct set of symptoms that feel different from a standard bladder infection. The pain typically sits in your flank, the area just below your ribs on either side of your spine. It can radiate around your side toward your lower abdomen. Unlike the pelvic pressure of a bladder infection, this pain is deeper and often feels like a dull, persistent ache that worsens when the area is touched or bumped.
Along with flank pain, most people develop a fever, often above 101°F (38.3°C). Shaking chills, nausea, and vomiting are common. You may also still have lower urinary tract symptoms like burning with urination, a frequent urge to go, or cloudy and foul-smelling urine, but these can sometimes be mild or absent altogether. The combination of fever plus back or side pain is the clearest signal that an infection has moved beyond the bladder.
How It Feels Different From a Bladder Infection
A bladder infection generally makes you feel uncomfortable but not sick. You notice burning, urgency, and maybe some lower abdominal pressure. A kidney infection makes you feel systemically unwell. You may feel exhausted, achy, and unable to eat. The fever and chills are a major distinguishing factor because simple bladder infections rarely cause them.
Another way to gauge the difference: with a bladder infection, you can usually power through your day. A kidney infection often puts you on the couch or in bed. If your symptoms started as a typical UTI and then escalated over a day or two to include back pain and fever, that progression strongly suggests the infection has spread upward.
Symptoms Can Look Different in Older Adults
In people over 75, kidney infections often present without the classic symptoms. Fever may be absent entirely. Flank pain may be vague or replaced by general abdominal tenderness. Instead, the most noticeable changes tend to be a decline in daily functioning, sudden confusion or worsening of existing dementia, weight loss, or unusual drowsiness. In someone with dementia, increased agitation or behavioral changes may be the only visible sign. These atypical presentations make kidney infections in older adults easier to miss and more dangerous as a result.
How Doctors Confirm the Diagnosis
If you go in with symptoms, your doctor will likely start with a physical exam that includes pressing or gently tapping on the area of your back just below your ribs. Pain when that spot is tapped is a strong indicator that the kidney is inflamed or infected.
From there, a urinalysis checks your urine for white blood cells and blood. Your body ramps up white blood cell production when fighting a bacterial infection, so elevated levels point toward infection. A urine culture is also typically ordered to identify the specific bacteria causing the problem. This helps your doctor choose the right antibiotic rather than guessing.
In more severe cases, or if you’re not improving with treatment, imaging like an ultrasound or CT scan may be used to check for blockages, abscesses, or structural problems in the urinary tract.
Why At-Home UTI Tests Aren’t Enough
Over-the-counter UTI test strips can detect signs of a urinary infection, like the presence of white blood cells or certain bacterial byproducts. But they can’t tell you where the infection is. A positive result on a home test strip means bacteria are present in your urine. It doesn’t distinguish between a mild bladder infection and a kidney infection that needs urgent treatment. If you have any systemic symptoms like fever, chills, flank pain, nausea, or vomiting, skip the home test and go directly to a doctor. Home strips are designed for otherwise healthy people with straightforward, uncomplicated UTIs.
Who Gets Kidney Infections
Kidney infections affect women far more often than men. Population data shows roughly 15 to 17 cases per 10,000 women annually in the United States, compared to 3 to 4 per 10,000 men. Women between 18 and 49 face even higher rates, around 28 cases per 10,000. At least 250,000 cases are diagnosed in the U.S. each year. The disparity comes down to anatomy: women have shorter urethras, giving bacteria a shorter path to the bladder and eventually the kidneys.
Risk factors beyond sex include a history of recurrent UTIs, pregnancy, kidney stones or other urinary tract obstructions, a weakened immune system, diabetes, and use of a urinary catheter.
What Treatment Looks Like
Kidney infections are treated with antibiotics, and most people start feeling better within 48 to 72 hours of starting them. Current guidelines from the Infectious Diseases Society of America recommend 5 to 7 days of treatment in many cases, though some people may need up to 14 days depending on severity and the type of bacteria involved. Your doctor will choose the antibiotic based on your urine culture results whenever possible.
Mild to moderate cases can often be treated at home with oral antibiotics, plenty of fluids, and rest. More severe infections, particularly those involving high fevers, persistent vomiting (which prevents you from keeping pills down), or signs of the infection spreading to the bloodstream, may require a hospital stay for intravenous treatment.
Warning Signs That Need Emergency Care
Most kidney infections respond well to antibiotics when caught early. But if the infection enters the bloodstream, it can progress to a condition called urosepsis, which is a medical emergency. Watch for a rapid heart rate, difficulty breathing, a weak pulse, inability to urinate, or feeling confused or disoriented. If you already have a diagnosed kidney infection and your symptoms suddenly worsen rather than improve, or if you develop any of these signs, get to an emergency room. Sepsis from a urinary source can escalate quickly, and early treatment dramatically improves outcomes.