Kidney infections are diagnosed through a combination of symptoms, a physical exam, urine tests, and sometimes blood work or imaging. Most cases can be confirmed with a urinalysis and urine culture, but your doctor will start by evaluating your symptoms and examining you to determine whether the infection has moved beyond the bladder and into the kidneys.
Symptoms That Point to a Kidney Infection
A kidney infection typically starts as a lower urinary tract infection that spreads upward. The hallmark signs that distinguish it from a simple bladder infection are fever, flank pain (pain in your upper back or side, below the ribs), and nausea or vomiting. These symptoms don’t always appear together, and they can range from barely noticeable to severe. Most people develop them over hours or over the course of a single day, though some have mild symptoms for days or even weeks before seeking care.
Fever isn’t always present, but when it is, it often exceeds 103°F (39.4°C). Flank pain can affect one side or both. You’ll likely still have the bladder symptoms you started with: burning during urination, frequent urges to go, cloudy or foul-smelling urine, and sometimes blood in the urine. The addition of high fever, back or side pain, and nausea is what signals the infection has reached the kidneys.
The Physical Exam
One of the most telling parts of the exam is a technique called costovertebral angle (CVA) tenderness testing. Your doctor places a flat hand on your back just below your lowest ribs, near where the kidneys sit, and thumps it with their other fist. This sends a vibration through the kidney. If you feel a sharp pain when they do this, it strongly suggests kidney involvement. If there’s no pain, a kidney infection becomes less likely. You can be standing, sitting, or lying down for this test.
Your doctor will also check your vital signs, particularly your temperature and heart rate, since fever and an elevated heart rate can indicate a more serious or systemic infection.
Urinalysis: The First Lab Test
A urinalysis is the quickest way to check for infection. This is a simple urine sample analyzed either with a dipstick or under a microscope. Two key markers stand out:
- White blood cells in urine. These immune cells show up when your body is fighting an infection in the urinary tract. Their presence is one of the strongest initial indicators.
- Nitrites. Many bacteria that cause UTIs convert a normal urine chemical called nitrates into nitrites. If nitrites show up on the dipstick, it’s a reliable sign of bacterial infection. However, some bacteria don’t produce this conversion, so a negative nitrite result doesn’t rule out infection.
A urinalysis provides fast results, often within minutes, but it can’t tell your doctor exactly which bacteria are causing the infection or which antibiotics will work. That’s where the urine culture comes in.
Urine Culture: Confirming the Diagnosis
A urine culture is the definitive test for confirming a kidney infection. Your urine sample is placed in a dish and monitored for bacterial growth over one to three days. The results identify the specific type of bacteria and test which antibiotics will kill it.
A count of 100,000 or more colony-forming units per milliliter of urine is the standard threshold for confirming a kidney infection. About 80% of women with pyelonephritis (the medical term for a kidney infection) meet this threshold. In some cases, lower counts can still indicate infection, especially if symptoms are clear and other test results align.
Because culture results take time, most doctors will start antibiotics based on your symptoms and urinalysis while waiting for the culture to come back. If the culture shows the bacteria are resistant to the initial antibiotic, your treatment can be adjusted.
Blood Tests for Severe Cases
Not everyone with a kidney infection needs blood work, but it becomes important when the infection appears severe or when there’s concern it may be spreading into the bloodstream. A complete blood count can reveal elevated white blood cells, confirming your body is fighting an active infection. Markers of inflammation, like C-reactive protein (CRP), rise significantly during kidney infections. CRP levels at or above 8 to 10 mg/L are considered high and suggest a meaningful inflammatory response.
Your doctor may also check kidney function through a blood test that measures creatinine, a waste product your kidneys normally filter out. Elevated creatinine levels can indicate the infection is impairing kidney function, which changes how aggressively the infection needs to be treated. Blood cultures, where a blood sample is checked for bacteria, may be drawn if sepsis is a concern.
When Imaging Is Needed
Most straightforward kidney infections don’t require imaging. If it’s your first episode, your symptoms are typical, and you start improving on antibiotics, a urinalysis and culture are usually enough. Imaging comes into play in specific situations: when symptoms persist or worsen after 72 hours of antibiotic treatment, when you have a history of kidney stones or prior kidney infections, or when you have conditions like diabetes, immune suppression, pregnancy, or anatomic abnormalities of the urinary tract.
A CT scan with contrast dye is the most accurate imaging option for kidney infections. It can reveal swelling of the kidney, fluid around the kidney, kidney stones, or an abscess that might be preventing recovery. Ultrasound is sometimes used as a first step, particularly during pregnancy when radiation from a CT scan should be avoided. It’s good at detecting blockages and fluid collections but less sensitive for spotting the subtle tissue changes of infection. MRI is another alternative for patients who can’t undergo CT scanning.
Telling It Apart From Similar Conditions
Several conditions mimic the symptoms of a kidney infection, and part of the diagnostic process involves ruling them out. Kidney stones can cause intense flank pain and blood in the urine, but they typically don’t cause fever unless an infection develops alongside them. A CT scan can quickly distinguish between the two, since stones are highly visible on imaging.
A simple bladder infection shares many urinary symptoms with a kidney infection but lacks the high fever, flank pain, and nausea. The CVA tenderness test and the presence of systemic symptoms like fever and chills help draw the line. In women, pelvic inflammatory disease can cause lower abdominal pain and fever, but urinalysis results will typically be normal since the infection involves the reproductive organs rather than the urinary tract.
If your symptoms overlap with multiple possible conditions, your doctor may order a broader set of tests to narrow things down. The combination of a positive urine culture, CVA tenderness, and systemic symptoms like fever is what clinches the kidney infection diagnosis.