Sun28November0440PM 30

A 14-year-old girl is admitted with headache, nausea and vomiting. She had previously been well but her symptoms deteriorated over the last 12 hours.

On admission, she is noted to be slightly confused with a temperature of 39 degrees centigrade, stiff neck positive Kernig's sign and a faint purpuric rash on the knees. Her blood pressure is 90/60 mmHg with a pulse of 120 beats per minute.

A diagnosis of meningococcal meningitis is confirmed following CT head scan and lumbar puncture. She is admitted to ITU and treated with IV cefotaxime 2 g tds and benzylpenicillin 2.4 g qds. She has a rather stormy admission requiring intubation, ventilation and hypotensive episodes. On day two of her admission, her urine output falls with hourly urine output of approximately 10 ml/hr.

Investigations reveal:

Haemoglobin 167 g/L (115-165)

White cell count 16.8 ×109/L (4-11)

Platelets 100 ×109/L (150-400)

Serum sodium 125 mmol/L (137-144)

Serum potassium 5 mmol/L (3.5-4.9)

Serum urea 6.7 mmol/L (2.5-7.5)

Serum creatinine 100 µmol/L (60-110)

Plasma osmolality 300 mosmol/Kg (278-305)

Urine osmolality 285 mosmol/Kg (350-1000)

Urine urea 120 mmol/L

Urine sodium 75 mmol/L

Why is this acute tubular necrosis and not pre-renal failure?

(Please select 1 option)

Her blood pressure is low

Her urine:plasma osmolality ratio is >1:1

Her urine:plasma urea ratio is elevated Incorrect answer selected

Her urine output falls

Her serum sodium is 125 mmol/L This is the correct answer

Explanation

In acute tubular necrosis (ATN), urine to plasma osmolality should be less than 1.1, urinary sodium excretion is typically more than 60 mmol/L and urinary urea excretion less than 160 mmol/L.

If this patient had a physiological oliguria, there would still be preservation of urine concentration, with low urinary sodium.

Both ATN and pre-renal failure can present with a fall in urine output. There is such a marked variation in urine urea concentration, that it is seldom used as a clinical guide.