Mixed Growth in a Urine Culture
A 26-year-old woman presents with dysuria, urinary frequency, and suprapubic discomfort for two days. She has no fever, rigors, flank pain, vomiting, or vaginal discharge. A clean-catch midstream urine specimen is collected after she is given verbal instructions. Dipstick testing shows leukocyte esterase 2+, nitrite negative, and blood 1+. After 24 hours of incubation, the culture grows three morphotypes: Escherichia coli 8 × 10^3 CFU/mL, coagulase-negative staphylococci 5 × 10^3 CFU/mL, and alpha-haemolytic streptococci 4 × 10^3 CFU/mL. The laboratory protocol reports this as “mixed growth; probable contamination”. The clinician asks whether the patient has a urinary tract infection and whether antibiotics should be started immediately. The patient is otherwise well and can return for a repeat sample tomorrow.
1. Interpret the dipstick and culture. Does this support a urinary tract infection?
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The leukocyte esterase is positive, indicating pyuria, but nitrite is negative, which is common with non-nitrate-reducing organisms and with frequent voiding. The culture shows three organisms, each at 4–8 × 10^3 CFU/mL, a total of 1.7 × 10^4 CFU/mL of mixed flora. In a symptomatic woman, a single uropathogen at 10^3 CFU/mL or more can be significant, but multiple organisms at low counts usually reflect contamination from skin or vagina. The combination does not support a single-organism UTI. However, symptoms and pyuria mean UTI cannot be excluded clinically; the culture is inadequate.
2. What should the laboratory do next and what collection advice is needed?
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Report mixed growth with probable contamination and do not perform susceptibility testing on mixed flora. Request a repeat clean-catch midstream specimen before antibiotics if the patient is stable. Give instructions: wash hands, separate the labia, clean the periurethral area with water, void a small amount, collect midstream into a sterile container, close it, label it, and deliver within 1–2 hours or refrigerate. If contamination persists and symptoms continue, a catheterised specimen may be needed. Do not collect from a bedpan or a urine bag.
3. When would antibiotics be justified before culture, and when is a low colony count significant?
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Start empirical antibiotics before culture if there is pyelonephritis, sepsis, pregnancy, immunocompromise, or severe symptoms; otherwise await a proper sample. A low colony count is significant when a single recognised uropathogen grows at 10^3 CFU/mL or more in a symptomatic woman, or at 10^5 CFU/mL or more in asymptomatic patients; thresholds differ in men, children, and catheterised patients. Mixed growth of three organisms at 10^3–10^4 CFU/mL is not treated as UTI. After treatment, repeat culture only if symptoms persist or recur.