Last updated on: June 3rd, 2024

Hi there!

You’re looking at a short reference article from Explain Medicine (one of four distinct learning formats available in Clinical Odyssey). Try it out, and have fun improving your clinical skills.

Acute pyelonephritis

Clinicals - History


Acute pyelonephritis (APN) is a common bacterial infection of the renal parenchyma that can be life-threatening if not promptly diagnosed and treated. Escherichia coli is the organism most often implicated.


Fever >100.4°F (38.0 °C) accompanied by chills is a characteristic feature. However, this may be mild or even absent in the elderly and immunocompromised. The pathophysiology involves endogenous pyrogens released by the underlying inflammatory process.

Flank pain

Flank pain or costovertebral pain is the other characteristic feature of APN. The pain is felt in these areas due to the retroperitoneal location of the kidneys.

Nausea and vomiting

Nausea and vomiting are common and can vary in intensity. These too are due to the systemic release of inflammatory mediators.

Lower urinary tract symptoms

Lower urinary tract symptoms include dysuria, frequency, and urgency. They occur if there is a co-existing (lower) urinary tract infection (UTI).


Complications include sepsis and abscess formation. Sepsis occurs when severe infection leads to an uncontrolled systemic inflammatory response. Renal and perinephric abscesses occur due to the local spread of infection. Emphysematous pyelonephritis is yet another life-threatening complication that can occur in diabetic or immunocompromised persons. Renal scarring and renal failure may also occur in recurrent/uncontrolled pyelonephritis.

Risk factors

Risk factors for the development of acute pyelonephritis include:

  1. a UTI within the previous 12 months,
  2. functional and structural genitourinary defects,
  3. pregnancy, particularly the 2nd and 3rd trimesters,
  4. diabetes mellitus, and
  5. recent instrumentation of the urinary tract.

Complicated pyelonephritis

The term complicated acute pyelonephritis is used if there is an anatomical or functional abnormality predisposing the patient to a greater risk of complications. This may be due to the presence of one or more of the following:

  1. uncontrolled diabetes mellitus,
  2. immunocompromised state,
  3. renal transplantation,
  4. structural abnormalities of the urinary tract,
  5. pregnancy, or
  6. hospital-acquired infections.

Want to continue reading?

Subscribe to Clinical Odyssey today.
  • Enjoy unlimited access to 700+ learning modules.
  • Safely improve your skills, anytime and anywhere.
  • Get answers to your follow-up questions from practicing physicians.
Learn more ➜