Kidney infections (pyelonephritis) are most often caused by ascending bacteria, especially E. coli. Obstruction from stones, prostate disease, congenital anomalies, pregnancy changes, and catheters raise risk. Presentations include fever, flank pain, and urinary symptoms; diagnosis uses urinalysis, culture, and imaging when needed. Treatment depends on severity and may require antibiotics and urologic drainage. Elderly and immunocompromised patients face higher risk of severe complications, including sepsis.
What is pyelonephritis?
Pyelonephritis is a bacterial infection of the kidney tissue and renal pelvis. It most often results from bacteria that ascend from the bladder through the ureters. In some cases the kidney is seeded from the bloodstream. Left untreated, pyelonephritis can cause kidney scarring, abscess, or life-threatening sepsis.
Common causes and risk factors
- Bacteria from the bowel: Enteric Gram-negative organisms, especially Escherichia coli, cause the majority of community-acquired kidney infections.
- Urinary obstruction: Kidney stones, an enlarged or inflamed prostate, ureteral strictures, or other blockages can trap urine and allow bacteria to multiply.
- Structural abnormalities: Congenital problems such as vesicoureteral reflux or abnormal ureteral insertion increase risk, especially in children.
- Pregnancy: Physiologic urinary stasis and ureteral dilation during pregnancy raise risk and can increase the chance of preterm labor when infection occurs.
- Catheters and instrumentation: Long-term urinary catheters and some urologic procedures increase risk of ascending infection.
How infections reach the kidney
Most kidney infections start as a lower urinary tract infection (cystitis) and travel upward. In a smaller number of cases, bacteria in the bloodstream localize to the kidney. Obstruction and foreign bodies (stones, catheters) provide surfaces where bacteria can persist, leading to recurrent or chronic infections.
Symptoms and diagnosis
Typical symptoms include fever, chills, flank pain or tenderness, nausea or vomiting, and urinary symptoms (frequency, urgency, burning). Older adults may present atypically with confusion or weakness.
Initial testing includes a urinalysis (showing pyuria and often blood) and a urine culture to identify the organism and antibiotic susceptibilities. Blood tests can show leukocytosis and, if severe, organ-dysfunction markers. Imaging (renal ultrasound or CT) is used when obstruction, abscess, or complications are suspected. In children and in recurrent infections, targeted imaging to look for reflux or congenital anomalies is commonly performed.
Treatment and complications
Uncomplicated outpatient pyelonephritis may be treated with oral antibiotics selected based on local resistance patterns and culture results. Severe infections, dehydration, high fever, or signs of systemic illness require hospitalization and intravenous antibiotics. When obstruction (for example from a stone or enlarged prostate) causes infection, urologic drainage or intervention is often necessary.
Complications include kidney scarring, renal abscess, recurrent infections, and sepsis. The elderly and people with weakened immune systems have higher risks of severe outcomes.
Prevention
Preventive steps include treating bladder infections promptly, managing stones and obstructing conditions, removing unnecessary catheters, and following prenatal screening recommendations in pregnancy. For people with recurrent infections, a urology evaluation can identify correctable problems.