Urinalysis – What Each Component Means

www.lifeinthefastlane.com has a great article describing the components of the humble urine dipstick and what we might learn from it. It is available here and I am going to borrow extensively from Dr Mike Cadogan’s work over the next few months but try to put a paediatric slant on it.

1) SPECIFIC GRAVITY (SG) – measures concentration of urine

Normal range varies by lab but roughly 1.005 to 1.030
< 1.005 – diabetes insipidus, fluid overload, pyelonephritis
> 1.030 – dehydration, glycosuria, SIADH
Falsely high in proteinuria, falsely low in alkaline urine

2) pH

Glomerular filtrate has a pH of about 7.4 which is acidified to about 6 by the time it is passed as urine.

Causes of alkaline urine (⇑pH)
Causes of acidic urine (⇓pH)
Old sample, vegetarian diet, salicylate
overdose, UTI, citrus fruit ++, low carb diet
Metabolic/respiratory acidosis, diarrhoea, high
protein diet, DKA, cranberries, malabsorption
Not a helpful test as can vary from 4.5 to 8.  Stones can form with either alkaline or acidic urine

3) NITRITES

Nitrites on a dipstick test has a positive predictive value of 96% ie. it is highly likely that the child has a UTI. But the test’s negative predictive value is not so good (around 70%) ie. some children still have a UTI even though they have no nitrites in their urine. Why?

  • ? only gram -ive bacteria convert nitrates to nitrites in urine; E coli, Proteus and Klebsiella are gram -ive, Enterococcus is not
  • Can take 4 hours for this conversion to take place. Babies don’t hold urine in their bladder for that long.

The current NICE UTI guideline recommends microscopy and culture to rule out UTI in children younger than 3 but suggests that dipstick urinalysis is enough in older children. They are currently looking at new evidence to see if the dipstick result (leucocytes and nitrites) can be “trusted” in younger children. Update due to be published this year.

4) LEUCOCYTES

  • Determines the presence of whole or lysed white cells in the urine (pyuria) by detecting leucocyte esterase activity.
  • A positive leucocyte esterase test correlates well with pyuria. BUT, pyuria does not necessarily indicate a UTI. The white cells may be increased because of infection elsewhere. NICE “do not do recommendation”: Do not test urine if the infant or child has an obvious alternative source of infection.
  • Conversely, a UTI diagnosis may be missed if a negative urinalysis dipstick is used to exclude UTI. Especially true in children less than 3 years old. NICE recommendation: if you suspect a UTI clinically, send urine for MC&S and do not rely on the dipstick result alone; we are supposed to diagnose a UTI if there is bacteriuria on microscopy, even without pyuria. Click here for further information on diagnosing UTI in children; it’s not quite as straight forward as you would hope.

5) BLOOD

  • Red or brown urine does not always mean blood
  • High false positive rate (eg. haemoglobinuria, myoglobinuria, concentrated urine, menstrual blood in the urine sample, rigorous exercise) so dipstick positive blood needs to be looked at under the microscope to accurately diagnose haematuria
  • False negative possible if specific gravity is < 1007
  • Significant haematuria is defined as ≥ 10 red blood cells (≥ 3 in adults) per high-power field in a properly collected and centrifuged urine specimen
  • Isolated microscopic haematuria in a well child only really needs further investigation after 3 positive samples over a period of a few months
  • Concomitant proteinuria, high BP or a palpable abdominal mass should be investigated promptly
  • Possible causes of haematuria in children:
    • UTI
    • Viral infections
    • Post streptococcal glomerulonephritis
    • Trauma
    • Henoch Schonlein Purpura
    • Wilm’s tumour (median age 3.5 years)
  • The Royal Children’s Hospital in Melbourne has a sensible, easy-to-follow guideline for the management of children with haematuria

6) PROTEIN

  • Normal daily protein excretion ≤ 150mg/24 hours or 10mg/100mL. In nephrotic syndrome >3.5g/day is excreted. “Trace” positive results = 10 mg/100 ml or about 150 mg/24 hours (the upper limit of normal).
  • Causes: transient or orthostatic (most common and benign), click here for summary of causes in children
  • False Positive: Concentrated or alkaline urine (pH >7.5), trace residue of bleach, NaHCO3, cephalosporins
  • False Negative: Dilute urine or acidic urine (pH <5)  Use spot, early morning urine testing for a protein/creatinine ratio if the urine dipstick test result is 1+ protein or more. A 24 hour collection is impractical
Dipstick protein reading Protein excretion gm/24 hours Protein excretion mg/dL
Negative <0.1 <10
Trace 0.1-0.2 15
1+ (and above is abnormal) 0.2-0.5 30
2+ 0.5-1.5 100
 3+ 2.0-5.0 300
4+ >5.0 >1000

7) KETONES

Ketones are not normally found in the urine. Produced by the liver as intermediate products of fatty acid metabolism, in normal states they will be completely metabolised. In “starvation” states eg. DKA or vomiting and reduced intake, fever, extreme cold and extreme exercise, the body metabolises increased fat to get the energy it needs to keep functioning. This results in ketonuria. ≥ ++ is abnormal.  We often see ketones in the urine of unwell children in the ED. When glucose is present at the same time in the urine, diabetic ketoacidosis is the likely diagnosis.

RESOURCES

http://lifeinthefastlane.com/investigations/urinalysis/
http://labtestsonline.org.uk/understanding/analytes/urinalysis/ui-exams?start=1
https://patient.info/doctor/urine-ketones-meanings-and-false-positives-pro

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