Intended implementation date: 5 October 2026.

Proposal

To increase the urine white blood cell (WBC) threshold used for routine urine culture from 10 x10⁶/L WBC to 40 x10⁶/L WBC for females age 12 years or older.

Process

Under the proposed process:

• Urine samples with < 10 x10⁶/L WBC will continue to be managed according to the existing laboratory protocol (no change).
• Urine samples from females aged 12 years or older with 10–39 x10⁶/L WBC will still be cultured and processed by the laboratory, but the organism identification and susceptibility result will not routinely be released.
  • Instead, the report will include a comment advising to contact the microbiology laboratory if the culture result will change clinical management.
• Urine samples with 40 x10⁶/L WBC or more will have culture results reported through the usual pathway (no change).
• Urine processing remains unchanged for males, patients <12 years, and where clinical details indicate agreed clinical exceptions (e.g. pregnancy, pyelonephritis, neutropenia, pre-urological procedure screening samples).

Background

The change will bring our Awanui Te Waipounamu laboratories more closely into line with the urine WBC thresholds used by other laboratories across Aotearoa New Zealand, most commonly 40-50 x10⁶/L WBC (Awanui Auckland, Awanui Wellington, Labplus Auckland, Pathlab). Current benchmarking indicates that our existing threshold of 10 x10⁶/L WBC makes our laboratories an outlier and results in culture reporting for a larger group of patients with only low-level pyuria.

Whilst pyuria is widely used as a screening marker for urinary tract infection (UTI), there is no universally accepted WBC threshold that optimally predicts clinically significant bacteriuria. While pyuria may be associated with UTI, lower levels of pyuria have limited specificity. Published evidence demonstrates considerable variation in thresholds used

internationally, and there remains inconclusive evidence that lower WBC thresholds improve patient outcomes.

Locally, our data has demonstrated that for patients with a urine sample submitted to the laboratory, a significant proportion (>50%) are not initially treated empirically at the time of the consultation. Of these patients not treated at the time, around 45% of antibiotic prescriptions for UTI occur after the urine culture report has been issued, usually 2-3 days after the sample has been submitted (unpublished data). It is not clear whether these prescriptions are clinically warranted, since empiric UTI treatment guidelines cover >93% of uropathogens and inappropriate treatment of asymptomatic bacteriuria is harmful.

Reporting organism identification and susceptibility results in patients with a low pre-test probability of infection may encourage clinicians to treat bacteriuria that is incidental to the patient’s presentation and potentially constitutes a misdiagnosis. Potential harms from unnecessary antibiotic treatment are therefore significant and wide ranging: misdiagnosis of the true underlying condition and incorrectly ascribing other conditions to UTI, adverse events such as allergic reactions, adverse drug effects, gastrointestinal adverse effects (including Clostridioides difficile infection), selection of resistant organisms and disruption of the patient’s microbiome).^1-6

Our laboratory data demonstrates that raising the WBC threshold for culture from 10 to 40 x10⁶/L WBC will affect 1.5% of samples and will improve test specificity.

The proposed threshold of 40 x10⁶/L WBC should be regarded as a pragmatic laboratory and antimicrobial-stewardship threshold which aims to balance:

• the need to retain sensitivity for clinically meaningful infection;
• the low specificity of low-level pyuria;
• the risk that reporting low-probability positive cultures will prompt unnecessary and harmful antibiotic treatment;
• alignment with comparator New Zealand laboratories; and
• the availability of a safety pathway through continued culture processing and clinician-requested release.

The proposed change is not driven by laboratory cost considerations, because these samples will continue to be processed during this pilot phase. Its primary benefit is clinical, with an expected reduction in inappropriate downstream management decisions.

Local empirical antibiotic treatment coverage

For uncomplicated lower urinary tract infection, treatment is usually empirical and should be guided primarily by compatible symptoms, patient factors and local guidelines rather than by routine culture.

According to the current local antibiogram, nitrofurantoin (the recommended first-line empirical agent) covers approximately 99% of relevant uropathogens whilst cefalexin (the recommended alternative) covers approximately 93%.^7,8

This high predicted coverage reduces the likelihood that routinely releasing a culture from the 10–39 x10⁶/L WBC group will be necessary for the initial management of an otherwise uncomplicated lower urinary tract infection.

Empirical treatment must still take account of patient factors (e.g. allergy, renal function, previous microbiology, suspected site and severity of infection etc).

Proposed report comment

Samples with 10–39 x10⁶/L WBC will have the following comment added to the report:

The urine WBC count is below the laboratory threshold of 40 x10⁶/L WBC for routine release of culture results, as low-level pyuria is not specific for UTI. If UTI symptoms persist, and antimicrobial resistance is a concern, please contact the microbiology laboratory with relevant clinical information to ask for the release of the culture result – email DN.MICRO@awanuilabs.co.nz (for Otago Southland) or NELSON.MICRO@awanuilabs.co.nz (for Nelson Marlborough).

Pilot period

A two-month monitored pilot is recommended, at the end of which a review of the process will be conducted. 

Feedback

We welcome all feedback on this proposal to scl.enquiries@awanuilabs.co.nz

References

1. Nicolle LE, Gupta K, Bradley SF, et al. Clinical practice guideline for the management of asymptomatic bacteriuria: 2019 update by the Infectious Diseases Society of America. Clinical Infectious Diseases. 2019;68(10)–e110.
2. Miller JM, Binnicker MJ, Campbell S, et al. Guide to Utilization of the Microbiology Laboratory for Diagnosis of Infectious Diseases: 2024 Update by the Infectious Diseases Society of America (IDSA) and the American Society for Microbiology (ASM) , Clinical Infectious Diseases, 2024;, ciae104, https://doi.org/10.1093/cid/ciae104
3. Tien H, Bond K, Hong W, Cronin K, Chan E. Exploring the association between the degree of pyuria and urinary tract infections. Microbiol Spectr. 2025 Apr;13(4):e0201524.
4. Trautner BW. Asymptomatic bacteriuria: when the treatment is worse than the disease. Nature Reviews Urology. 2012;9(2):85–93.
5. Petty LA, Vaughn VM, Flanders SA, et al. Risk factors and outcomes associated with treatment of asymptomatic bacteriuria in hospitalized patients. JAMA Internal Medicine. 2019;179(11):1519–1527.
6. Leis JA, Rebick GW, Daneman N, et al. Reducing antimicrobial therapy for asymptomatic bacteriuria among noncatheterized inpatients: a proof-of-concept study. Clinical Infectious Diseases. 2014;58(7):980–983.
7. Awanui Labs Community Urine antibiogram. Available at: https://fl-healthscope-media.s3.amazonaws.com/lab-sites/uploads/sites/3/2026/02/Awanui_urine_antibiogram.pdf
8. Te Whata Kuru National Antibiotic Guidelines. Available at: https://tewhatakura.nz/guidelines