• Acute pyelonephritis is diagnosed in a person with a proven urinary tract infection who has loin pain and/or fever. There are no clinical features or routine investigations that conclusively distinguish acute pyelonephritis from cystitis.
• Suspect acute pyelonephritis in people with loin pain and/or fever.
• Dipstick test the urine for leucocyte esterase and nitrite for evidence of a urinary tract infection (UTI). For further information, see Dipstick testing.
• If both dipstick tests are negative, a UTI is unlikely.
• If the leucocyte esterase test alone is positive, a UTI is moderately likely.
• If the nitrite test is positive, with or without a positive leucocyte esterase test, a UTI is highly likely.
• Consider and exclude other causes of loin pain and/or fever (particularly if both dipstick tests are negative) including:
• Pelvic inflammatory disease.
• Appendicitis.
• Renal calculi.
• Send a midstream (or catheter) specimen of urine for culture and sensitivity.
• A final diagnosis of acute pyelonephritis is made in people with loin pain and/or fever if:
• A UTI is confirmed by culturing a urinary pathogen from the urine, and
• Other causes for symptoms have been excluded.

Urinary nitrite and leucocyte esterase

• People with acute pyelonephritis have increased numbers of white blood cells and bacteria in their urine.
• Nitrite is produced by most urinary pathogens in contact with urine. A positive nitrite test is strongly suggestive of a urinary tract infection (UTI). A negative result occurs when:
• No pathogens are present.
• Pathogens are present, but were not in contact with urine long enough to produce detectable levels of nitrite.
• Pathogens that do not produce nitrite are present.
• Leucocyte esterase is present in white blood cells.
• A positive leucocyte esterase test occurs when sufficiently high levels of white blood cells are present in urine, as a response to either infection or contamination.
• A negative leucocyte esterase test occurs either because there is no infection present or because infection is present but the numbers of white blood cells are insufficient to produce a positive result.

Interpretation of urine dipstick tests in people with urinary tract symptoms

• Nitrite positive, and leucocyte esterase positive or negative — over 90% will have a UTI.
• Nitrite negative and leucocyte positive — 50% will have a UTI.
• Nitrite negative and leucocyte negative — 5% will have a UTI.

[Ramakrishnan and Scheid, 2005; COMPASS, 2007]

Dipstick tests for nitrite and leucocyte esterase

• Experts recommend urinary dipstick tests to detect nitrite and leucocyte esterase to help distinguish people with acute pyelonephritis from those with similar symptoms and signs who do not have a UTI [SIGN, 2006; COMPASS, 2007].
• Although dipstick tests are not highly sensitive and specific, they help support decisions to start immediate treatment for pyelonephritis or investigate for other causes of symptoms and signs.

Microbiological examination of urine for all people with suspected pyelonephritis

• Experts recommend sending a midstream or catheter specimen of urine for all people with suspected acute pyelonephritis to [SIGN, 2006; HPA and Association of Medical Microbiologists, 2008] to:
• Confirm the diagnosis.
• Guide an appropriate change of treatment if the infection is not sensitive to empirical antibiotics.

Clinical features suggestive of infection localized to the upper urinary tract

• In the absence of more sensitive clinical features or practical clinical tests, experts recommend diagnosing acute pyelonephritis based on evidence of a UTI in a person with loin pain or a temperature over 38°C [Tomson, 2003].