Lower urinary tract infections (UTIs) involve infection in the bladder, causing cystitis (inflammation of the bladder).
Pyelonephritis refers to inflammation of the kidney resulting from bacterial infection. The inflammation affects the kidney tissue (parenchyma) and the renal pelvis (where the ureter joins the kidney).
Urinary tract infections are more common in girls, where the urethra is much shorter.
The primary source of bacteria for urinary tract infections is faeces. Intestinal bacteria, such as Escherichia coli (E. coli), Klebsiella pneumoniae and enterococci, can easily journey to the urethral opening from the anus.
Presentation
The presentation in infants is non-specific:
- Fever
- Lethargy
- Irritability
- Vomiting
- Poor feeding
- Urinary frequency
Signs and symptoms in older infants and children are more specific:
- Abdominal pain, particularly suprapubic pain
- Dysuria (painful urination)
- Urinary frequency
- Urinary urgency
- Urinary incontinence
- Nocturnal enuresis (bedwetting)
- Fever
- Vomiting
Upper Urinary Tract Infection
The diagnosis of an acute upper urinary tract infection (pyelonephritis) is made if there is either:
- Fever over 38°C
- Loin pain or tenderness
TOM TIP: It is worth remembering fever and loin pain are the two criteria for diagnosing upper urinary tract infections. The distinction between upper and lower UTIs is crucial as it determines the management.
Urine Dipstick
The ideal urine sample is a clean catch sample, avoiding contamination. This can be tricky in younger children and babies, particularly girls. It often involves the parent sitting with the infant without a nappy and a urine pot held ready to catch the sample. A clean catch sample helps avoid unreliable microbiology results. Specially designed urine collection pads may be helpful.
Nitrites on a dipstick test suggest bacteria in the urine. Gram-negative bacteria (e.g., E. coli) break down nitrates (a normal waste product in urine) into nitrites.
Leukocytes are white blood cells. It is normal to have a small number of leukocytes in the urine, but a significant rise can result from an infection or other cause of inflammation. Leukocyte esterase (a product of leukocytes) is tested on a urine dipstick, indicating the number of leukocytes in the urine.
Red blood cells in the urine indicate bleeding. Microscopic haematuria is where blood is seen on a urine dipstick but not seen when looking at the sample. Macroscopic haematuria is where blood is visible in the urine. Haematuria is a common sign of infection but can also be present with other causes, such as nephritis and haemolytic uraemic syndrome.
Positive nitrites and leukocytes indicate a urinary tract infection. Negative nitrites and leukocytes exclude a UTI. When one is positive and the other is negative, this is less convincing of an infection, and a sample should be sent for further testing.
A midstream urine (MSU) sample sent for microscopy, culture and sensitivity (MC&S) testing will determine the infective organism and the antibiotics that will be effective in treatment. This should ideally be sent before starting antibiotics.
Management
Children under 3 months with a fever should have a full septic screen (including bloods, blood cultures, urine culture and considering a lumbar puncture) and start immediate IV antibiotics (e.g., ceftriaxone and amoxicillin).
Children with features of sepsis or upper urinary tract infection will require inpatient treatment with IV antibiotics.
Follow local guidelines and take into account urine culture results when deciding on an antibiotic. Uncomplicated lower urinary tract infections are treated with 3 days of oral antibiotics. Typical choices in children are:
- Trimethoprim (first-line if low risk of resistance)
- Nitrofurantoin (first-line)
- Cefalexin
- Amoxicillin
Further Imaging
Further imaging is required in specific scenarios to identify the underlying cause and renal damage.
NICE (2019) recommends an ultrasound scan for:
- All children under 6 months with their first UTI (within 6 weeks)
- Recurrent UTIs (within 6 weeks)
- Atypical UTIs (e.g., very unwell or atypical organisms) (during the illness)
A DMSA (dimercaptosuccinic acid) scan is recommended 4-6 months after the infection to assess for damage from recurrent or atypical UTIs. A radioactive material (DMSA) is injected, and a gamma camera is used to determine how well the kidneys take up the material. Patches of kidneys that do not take up the material suggest scarring.
A micturating cystourethrogram (MCUG) is used to test for vesicoureteral reflux (VUR) in infants under 6 months with recurrent or atypical UTIs. It is also considered where there is a family history of vesicoureteral reflux, dilatation of the ureter (on ultrasound) or poor urinary flow. It involves catheterising the child, injecting contrast into the bladder and taking a series of x-rays to determine whether the contrast is refluxing into the ureters. Prophylactic antibiotics are given for 3 days around the test to reduce the risk of infection.
Vesicoureteral Reflux
Vesicoureteral reflux (VUR) involves urine flowing back into the ureters from the bladder. This predisposes patients to develop upper urinary tract infections and subsequent renal scarring. It is diagnosed using a micturating cystourethrogram (MCUG).
Less severe VUR may resolve as the child gets older. Management depends on the severity and may involve:
- Avoiding constipation
- Avoiding an excessively full bladder
- Prophylactic antibiotics
- Surgical input from paediatric urology
Last updated March 2025
Now, head over to members.zerotofinals.com and test your knowledge of this content. Testing yourself helps identify what you missed and strengthens your understanding and retention.
How to Learn Medicine Course