Sterile pyuria is the presence of elevated numbers of white cells (>10 white cells/mm³) in urine which appears sterile using standard culture techniques:

• Sterile pyuria is not an uncommon laboratory finding.¹
• Sterile pyuria is often found in female patients with symptoms of urinary tract infection (UTI). However, these results may be misleading for various reasons:
  • Standard laboratory culture conditions may not be optimal for growth of atypical organisms.
  • Laboratory may not report significant growth either because it was not a single organism or recognised urinary pathogen.
  • Fewer than 100,000 colony-forming units (cfu) per mL reported - e.g. it may be that urine was diluted by high fluid intake or an organism may be slow-growing. Studies have shown that approximately half of women presenting with symptoms and counts of 100 -10,000 cfu/mL have genuine bladder infections.
• The presence of pyuria increases the significance of a low bacterial count in the urine.
• Cell count per high power field is inaccurate and use of a counting chamber or similar gives more accurate results.

Causes of sterile pyuria

• A recently (within last 2 weeks) treated urinary tract infection (UTI) or inadequately treated UTI.
• UTI with 'fastidious' organism (an organism that grows only in specially fortified artificial culture media under specific culture conditions), e.g. Neisseria gonorrhoeae.
• Renal tract tuberculosis, chlamydial urethritis.²
• False negative culture due to contamination with antiseptic.
• Contamination of the sample with vaginal leukocytes.
• Interstitial nephritis: sarcoidosis (lymphocytes not neutrophils).
• Urinary tract stones.
• Renal papillary necrosis: diabetes, sickle cell disease, analgesic nephropathy.
• Urinary tract neoplasm, including renal cancer and bladder cancer.
• Polycystic kidneys.
• Interstitial cystitis.
• Prostatitis.
• Other reported associations include appendicitis, systemic lupus erythematosus and Kawasaki disease.

Investigations

• Urinalysis: initial test to identify likely infection but a urine sample needs to be sent to the laboratory. See separate article Urine Dipstick Analysis. Positive nitrite test +/- positive leukocyte esterase test. Haematuria and proteinuria occur in urinary tract infection (UTI) but are also present in other conditions.
• Urine microscopy, culture and sensitivities; ask the laboratory to culture under conditions allowing identification of fastidious or slow-growing organisms.
• Consider the possibility of sexually transmitted disease; take a sexual history and consider sending swabs for chlamydia and N. gonorrhoeae.
• Polymerase chain reaction (PCR) testing of sterile pyuria has been recommended for the detection of Chlamydia trachomatis, mycoplasma and ureaplasma infections.³
• Always consider tuberculosis; culture for AFBs (3 early morning urine samples).
• With urine obtained direct from the bladder, any organism grown is significant and should be treated with a prolonged course of appropriate antibiotics.
• Cystoscopy may be required to exclude non-infective causes.

Management

• Management of any identified underlying cause.

Document references

1. Dieter RS; Sterile pyuria: a differential diagnosis. Compr Ther. 2000 Fall;26(3):150-2. [abstract]
2. Tayal SC, Pattman RS; Sterile pyuria: consider chlamydial infection. Br J Clin Pract. 1996 Apr-May;50(3):166-7. [abstract]
3. Nassar FA, Abu-Elamreen FH, Shubair ME, et al; Detection of Chlamydia trachomatis and Mycoplasma hominis, genitalium and Adv Med Sci. 2008;53(1):80-6. [abstract]

Acknowledgements