Pyelonephritis (kidney infection)

What is pyelonephritis?

Pyelonephritis is a bacterial infection of one or both kidneys. Acute pyelonephritis develops suddenly and is the most common form, while chronic pyelonephritis is a long-term condition that can develop after repeated kidney infections, particularly when structural problems such as vesicoureteral reflux or urinary obstruction are present.

Pyelonephritis is distinct from a lower urinary tract infection (cystitis), which is confined to the bladder and does not involve the kidneys. The distinction is important because pyelonephritis requires longer antibiotic courses and carries a higher risk of complications.

Symptoms of pyelonephritis

Acute pyelonephritis typically presents suddenly, often over hours to a few days. Common symptoms include:

• High fever with rigors and chills

• Flank pain or tenderness, typically one-sided, below the ribs and above the hip

• Nausea and vomiting

• Burning or pain during urination

• Frequent or urgent need to urinate

• Cloudy, dark, or foul-smelling urine

• General malaise, fatigue, and muscle aches

• In severe cases: very high fever, confusion, and signs of sepsis requiring emergency care

Chronic pyelonephritis may cause more subtle symptoms including persistent low-grade flank discomfort, recurrent urinary tract infections, high blood pressure, and progressive fatigue from declining kidney function.

Important: Fever with flank pain and urinary symptoms, particularly in someone who is elderly, pregnant, immunocompromised, or diabetic, requires prompt medical assessment. Delayed treatment significantly increases the risk of serious complications including urosepsis.

What causes pyelonephritis?

Pyelonephritis results from bacterial ascent through the urinary tract to the kidney. Contributing causes include:

• Escherichia coli is responsible for approximately 80% of cases, ascending from the bowel flora through the urethra and bladder to the kidney.

• Female anatomy the shorter female urethra significantly increases the risk of bacterial ascent compared to men.

• Urinary tract abnormalities including vesicoureteral reflux, kidney stones, ureteral stricture, and an enlarged prostate impair normal urine flow and create conditions that favour bacterial ascent and persistence.

• Urinary catheterisation and urological instrumentation introduce bacteria directly into the urinary tract.

• Diabetes impairs immune function, reduces bladder sensation from neuropathy, and promotes bacterial growth in glucose-rich urine.

• Pregnancy causes ureteral dilatation from hormonal and mechanical effects, increasing the risk of ascending infection.

• Immunosuppression from medication or underlying disease reduces the ability to clear urinary tract bacteria before they ascend.

• Previous urinary tract infections particularly recurrent cystitis, increase the risk of ascending pyelonephritis over time.

Acute pyelonephritis is usually diagnosed from symptoms together with urine testing. Blood tests and imaging may be used to assess the severity of the infection, identify complications, or look for underlying structural problems.

Abdominal MRI Elfcare’s full-body MRI directly images the kidneys to find swelling, scarring, or abscesses. It also identifies underlying causes, such as structural issues, that make you more likely to get infections. If you have recurring infections, the MRI shows if permanent shrinkage or damage has occurred.

Urine testing a urine dipstick, microscopic examination, and urine culture are the cornerstone investigations for suspected pyelonephritis. Urine culture helps identify the causative bacteria and guides antibiotic treatment.

Blood tests assess the severity of infection, systemic inflammatory response, and kidney function. Relevant markers in Elfcare's panel include:

• CRP: a high level confirms a strong inflammatory response; tracking it shows if your treatment is working.

• White blood cell count: confirms an active bacterial infection and assesses its severity.

• Creatinine, cystatin C, and eGFR: measures how well your kidneys are filtering blood during the infection.

• Urea: reflects nitrogen waste accumulation and provides additional kidney function context.

• Electrolytes: checks if the infection is disrupting your body’s mineral balance.

• HbA1c and glucose: identifies diabetes, which is a big risk factor for severe or repeat kidney infections.

• Albumin: low levels can signal a weakened immune system, making it harder to fight the infection.

• Haemoglobin and ferritin: checks for anemia, which often accompanies chronic kidney issues from repeat infections.

Why early detection matters

Most people with acute pyelonephritis recover fully when it is treated promptly with appropriate antibiotics. Without treatment, the infection can spread, increasing the risk of complications such as kidney abscesses or bloodstream infection. Recurrent infections or underlying structural problems, such as kidney stones or urinary obstruction, can increase the risk of long-term kidney damage, making further investigation important when infections keep returning.

How Elfcare can help

Elfcare's abdominal MRI includes both kidneys as part of the standard abdominal examination. It can identify structural abnormalities, scarring, abscesses, and other findings that may help explain recurrent infections or changes in kidney function.

Our blood panel covers CRP, full blood count, kidney function, glucose, and metabolic markers, providing a comprehensive picture of infection severity and the systemic factors most relevant to pyelonephritis risk and management.

If our tests identify findings consistent with pyelonephritis or related kidney pathology, we take care of further diagnostics or refer you to the appropriate specialist.