T4-L3: Laboratory Tests of Renal Function

Question 1

Define anuria, oliguria and polyuria.

Answer 1

Anuria - Loss of urine output <100mL

Oliguria - The minimum amount of urine you need to produced to get rid of the toxins by-poducts and solutes. This is defined as 400mL.

Polyruria - >3000mL

Question 2

How do we classify renal failure? What are the causes of the different types?

Answer 2

Pre-renal renal failure: Insufficient blood flow to the kidneys can cause acute pre-renal kidney failure. Can be due to an MI for example. Other causes include hypotension, dehydration and haemorrhage.

Renal-renal failure: This is intrinsic renal failure. Examples causes include acute tubular necrosis.

Post-renal Renal failure: Due to a loss of urinary output e.g. ureteral obstruction through an enlarged prostate or bladder cancer/

Question 3

List the laboratory tests we can do to asses renal function (11).

Answer 3

• Glomerular Filteration Rate
• eGFR
• Urine output volume
• Serum creatinine
• Urea
• Plasma urea
• Creatine clearance
• Urine electrolytes - e..g sodium
• Urine glucose
• Urine protein levels
• Haematuria

Question 4

What is the normal reference range for plasma urea?

Answer 4

3-8mmol/L

Question 5

Give causes of raised plasma urea.

Answer 5

Plasma urea is a sensitive non-specific index of illness. High protein levels of breakdown will lead to increased plasma urea levels.

Increased plasma urea can be caused by:

• Trauma
• Changes in kidney reabsorption and excretion
• GI bleed
• Renal hypo perfusion - Urea is filtered at the glomerulus with 40% reabsorption. If tubular flow decreases then more is absorbed leading to an amplifier affect. Tubular flow rare is slow when there is renal hypoperfusion.
• Chronic renal impairment
• Chronic Kidney Disease
• Post-renal obstruction calculus tumour (as it slows the filtration down due to back pressure)

Question 6

What is the reference range for plasma creatinine?

Answer 6

It has a wide reference range 50-140umol/L. Very little is reabsorbed. As GFR decreases, there is a rise in concentration. Plasma creatinine is not proportional to renal damage. Change within an individual patient is usually more important than the absolute value.

Question 7

Why do we often see increasing HTN in patients with a failed kidney?

Answer 7

Hypertension is become RAAS is affecting the vascular system directly. Some patients with a dead kidney need to the kidney taking out as they are producing aldosterone and the hypertension needs curing. The nephrons are unable to reabsorb the sodium leading to urine sodium concentrations greater than 40mmol/L.

Question 8

What are plasma normal sodium levels?

Answer 8

135-145 mmol/L