Acute pyelonephritis

Acute pyelonephritis is an acute inflammation of the kidney and its pelvis, usually caused by an ascending urinary tract infection.

Note that pyelonephritis in the presence of urinary tract obstruction is a urological emergency. This should be diagnosed and treated rapidly as there may be rapid progression to septicaemia and shock.

Acute pyelonephritis is a bacterial infection needing treatment with an antibiotic that reaches therapeutic concentrations in the kidney.

Gram-negative bacteria are the most common causative pathogens in acute pyelonephritis, with Escherichia coli (E. coli) causing 60 to 80% of uncomplicated infections

• other gram negative pathogens include Proteus mirabilis (responsible for about 15% of infections) as well as Klebsiella (approximately 20%), Enterobacter and Pseudomonas species
• less commonly, grampositive bacteria such as Enterococcus faecalis, Staphylococcus saprophyticus and Staphylococcus aureus may be seen

Antibiotics that don't achieve adequate levels in renal tissue, such as nitrofurantoin, fosfomycin and pivmecillinam, are to be avoided (1)

Nationally for England, resistance of E. coli (the main causative organism of acute pyelonephritis) in laboratory-processed urine specimens to the following antibiotics is:

• cefalexin: 9.9% (varies by area from 8.1 to 11.4%)
• ciprofloxacin: 10.6% (varies by area from 7.8 to 13.7%)
• co-amoxiclav: 19.8% (varies by area from 10.8 to 30.7%)
• trimethoprim: 30.3% (varies by area from 27.1 to 33.4%)

Reference:

• NICE (October 2018). Pyelonephritis (acute): antimicrobial prescribing