A kidney infection typically announces itself with a combination of fever, pain in your back or side, and feeling noticeably sick, not just uncomfortable. If you already have bladder infection symptoms like burning when you pee and suddenly develop a fever, chills, or deep pain beneath your ribs, the infection has likely traveled upward to one or both kidneys. This distinction matters because kidney infections need prompt medical treatment to prevent serious complications.
The Key Symptoms of a Kidney Infection
Kidney infections produce two categories of symptoms: the urinary symptoms you might already recognize from a bladder infection, plus systemic symptoms that affect your whole body. It’s the systemic symptoms that set a kidney infection apart.
The hallmark signs include:
- Fever above 38°C (100.4°F), often with chills or shivering episodes called rigors
- Pain in your flank, back, or groin, typically on one side, located below the ribs and above the hips
- Nausea or vomiting
- Fatigue and a general feeling of being unwell
- Cloudy or foul-smelling urine, sometimes with a pink, red, or brown tint from blood
You may also have burning during urination, urgency, and frequent trips to the bathroom, but these lower urinary symptoms aren’t always present with a kidney infection. Some people skip the classic bladder symptoms entirely and go straight to fever and flank pain. In older adults, the symptoms can be vaguer: confusion, general deterioration, or abdominal discomfort without obvious urinary complaints. Children under two may show only a high temperature with no other clear signs.
How Kidney Pain Differs From Back Pain
Many people searching for kidney infection symptoms are really trying to figure out whether the pain they feel is coming from their kidneys or their muscles. The location and behavior of the pain are the best clues.
Kidney pain is felt in the flank area, which sits on either side of your spine just beneath the rib cage. It tends to stay in one spot, though it can spread toward the lower abdomen or inner thighs. Crucially, it does not get better or worse when you shift positions or move around, and it won’t improve on its own without treatment. Musculoskeletal back pain, by contrast, is usually a dull ache or stiffness that changes with movement. You can often find a position that relieves it, and it commonly radiates down the legs if nerves are involved.
If tapping gently on your back just below the ribs produces a sharp, deep tenderness, that’s a classic sign pointing toward the kidneys rather than the muscles.
How a Kidney Infection Differs From a Bladder Infection
A bladder infection (cystitis) stays local. You feel it in your lower abdomen: burning when you pee, a constant urge to go, a sense that your bladder never fully empties. You might notice lower abdominal cramping or blood in your urine. What you typically don’t get with a simple bladder infection is a fever, chills, or pain high up in your back.
A kidney infection layers systemic illness on top of those local symptoms. The fever, nausea, and flank pain signal that bacteria have moved from the bladder up the ureters into the kidney itself. Clinicians categorize the severity in a straightforward way: mild pyelonephritis involves fever, flank pain, and nausea. Moderate to severe cases add shaking chills, dehydration, and vomiting. Severe cases can progress to dangerously low blood pressure and organ dysfunction.
The practical takeaway: if your symptoms are all below the belt (burning, urgency, frequency) you likely have a bladder infection. The moment you develop a fever or pain in your side or back, think kidney infection.
Who Is More Likely to Get One
Most kidney infections start as bladder infections that aren’t treated quickly enough, so anything that increases your risk of UTIs also raises your risk of a kidney infection. Women get kidney infections more often than men because of shorter urethras, which give bacteria a shorter path to the bladder.
Pregnancy is a notable risk factor. The growing uterus compresses the ureters and bladder, the bladder doesn’t empty as efficiently, and hormonal changes make urine less acidic and higher in sugars and protein. All of these shifts create a friendlier environment for bacteria. About 5 to 10 percent of pregnant women have bacteria in their urine with no symptoms at all, and if that silent infection goes untreated, it can progress to a kidney infection.
Other factors that raise risk include kidney stones or any structural blockage in the urinary tract, a weakened immune system, use of a urinary catheter, and conditions like an enlarged prostate that prevent the bladder from draining completely.
How Doctors Confirm a Kidney Infection
Diagnosis usually starts with your symptoms and a urine test. A urinalysis checks for white blood cells (a sign your body is fighting an infection) and blood in the urine. A urine culture identifies the specific bacteria causing the infection, which helps your doctor choose the right antibiotic.
In many cases, the combination of fever, flank pain, and a positive urinalysis is enough for a confident diagnosis. If the infection is severe, recurrent, or not responding to treatment, imaging such as an ultrasound or CT scan may be used to look for kidney stones, structural abnormalities, or abscesses.
What Treatment Looks Like
Kidney infections require antibiotics. Current guidelines recommend 5 to 7 days of treatment for most cases, though some people need up to 7 days depending on the specific antibiotic used. Your doctor will choose the antibiotic based on the bacteria identified in your urine culture. If you’re vomiting and can’t keep pills down, or if you’re severely dehydrated, you may need IV fluids and antibiotics in a hospital until you stabilize enough to switch to oral medication.
Most people start feeling noticeably better within 48 to 72 hours of starting the right antibiotic. Finishing the full course is important even after symptoms fade, because stopping early can allow resistant bacteria to survive and cause a relapse. If your symptoms aren’t improving after two or three days of treatment, contact your doctor, as the bacteria may be resistant to the antibiotic you’re taking.
Warning Signs That Need Emergency Care
A small percentage of kidney infections progress to a bloodstream infection. This is a medical emergency. Get to an emergency room if you experience a rapid heart rate or heart palpitations, difficulty breathing, an inability to urinate, a very low body temperature (below 36°C/96.8°F), or confusion and altered consciousness. These symptoms suggest the infection has spread beyond the kidney, and treatment needs to happen fast. Feeling feverish with chills alongside any of these signs is especially concerning.