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A 4-month-old girl is brought to the emergency department with fever (38.9 C), irritability, poor feeding, and two episodes of vomiting over the last 24 hours. Examination is unremarkable except for mild dehydration; there is no obvious focus of infection. A bag urine shows leucocytes and nitrites.
Questions
a) What is your diagnostic concern and the SINGLE most important next investigation? (2 marks)
The concern is a urinary tract infection — infants under 3 months (and considered under 3 years) with fever without an obvious source must have urine tested, because UTI is common and easily missed.
The single most important next step is a properly collected urine sample for culture — suprapubic aspiration (SPA) (gold standard in infants) or bladder catheterisation. A bag urine is NOT diagnostic (high contamination); its positive result must be confirmed by SPA/catheter.
b) State the urine culture thresholds that confirm UTI by collection method. (3 marks)
- SPA: ANY growth of a single organism is diagnostic (near-zero contamination).
- Catheter sample: single organism at 10^4 to 10^5 CFU/mL.
- Clean-catch: single organism at over 10^5 CFU/mL (over 10^4 CFU/mL with pyuria is also supportive).
- Bag/pad urine: NOT diagnostic; useful only if culture is negative (rules out). A positive must be confirmed by SPA/catheter/clean-catch.
c) How would you manage this child acutely? (3 marks)
- Obtain urine (SPA/catheter) for culture BEFORE antibiotics if possible, but do not delay treatment if she is septic.
- Because she is under 3 months / young infant and mildly dehydrated with vomiting, admit and give IV antibiotics active against E. coli — e.g. IV ceftriaxone 50 mg/kg once daily (or co-amoxiclav/gentamicin).
- Treat as pyelonephritis (upper UTI) for 7 to 10 days (high fever/vomiting/systemic illness); switch to oral when afebrile and improving.
- Supportive: IV fluids to correct dehydration, antipyretics.
- Renal ultrasound during the acute illness (infant under 6 months / atypical).
d) Outline the post-UTI investigation pathway for a child with a RECURRENT or ATYPICAL UTI. (2 marks)
- Renal ultrasound: anatomy and dilatation — all under 6 months, and any age for atypical/recurrent UTI.
- DMSA renal cortical scintigraphy 4 to 6 months after the upper/recurrent UTI — detects permanent renal scarring.
- MCUG (micturating cystourethrogram) — selectively, for atypical/recurrent UTI, to grade vesicoureteric reflux (VUR) and (in boys) exclude posterior urethral valves.
Rationale: untreated/recurrent pyelonephritis, especially with high-grade VUR, causes permanent renal scarring leading to hypertension and CKD, so long-term BP and urinalysis surveillance is required if scarring is confirmed.