16 - Tests of renal function and hydration status

What are the core key functions of the kidney?

- Fluid balance

- Acid-base balance

- Removal of waste products from the blood

- Maintenance of blood pressure by balancing sodiun, potassium and water (RAAS)

- Bone mineralisation (1,25 hydroxy vit D)

- RBC production (erythropoetin)

What are some pre-renal conditions?

- Heart failure

- Sepsis

- Blood loss

- Dehydration

- Vascular occlusion

What are some renal conditions?

- Glomerulonephritis

- Small-vessel vasculitis

- Acute tubular necrosis

- Interstitial nephritis

- Prolonged hypertension

What are some post-renal conditions?

- Urinary stones

- Benign prostatic enlargement

- Prostate cancer

- Cervical cancer

What is the key symptom of renal impairment?

Anuria

What is oliguria?

Decreased urine output

What volume is polyuria?

> 3000ml/24hrs

What is typical urine output?

750-2000ml/24hrs

What volume is oliguria?

<400ml/24hrs

What do renal function tests evaluate?

- Sodium

- Potassium

- Urea

- Creatinine

- eGFR

What is the glomerular filtration rate?

Volume of filtrate formed per minute by both kidneys

Describe plasma urea test:

- Quick, simple measurement

- Can be influenced by a high protein meal

- Range 3-8mmol/L

- Sensitive but non specific

Describe urea excretion test:

- Filtered at glomerulus but 40% reabsorbed in renal tubules normally

- Tubular flow rate increases urea absorption (hypoperfusion)

What factors affect urea?

- GI bleed

- Surgery, infection and trauma

- Drugs (steroids and tetracyclines)

- Cancer

- High protein diet

- Decreased excretion

What is eGFR?

Estimated GFR (based upon creatinine)

Describe plasma creatinine:

- Range 49-90micromole/L

- Increases after GFR has significantly decreased

Describe creatinine clearance:

- Urine is collected for 24 hours

- Volume, plasma creatinine and urine creatinine used too

How many stages are there in chronic kidney disease?

5 stages

What is stage 1 CKD?

Kidney damage with normal or increased GFR (>90)

What is stage 2 CKD?

Kidney damage with mild decrease in GFR (60-89)

What is stage 3 CKD?

Kidney damage with moderate decrease in GFR (30-59)

What is stage 4 CKD?

Kidney damage with severe decrease in GFR (15-29)

What is stage 5 CKD?

End stage kidney failure with a very severe decrease in GFR (<15 or on dialysis)

How many stages are there in acute kidney injury?

3 stages

What is stage 1 AKI?

- Serum creatinine of 1.5-1.9 times baseline within 7 days

- Urine output of <0.5ml/kg/hr for 6-12 hours

What is stage 2 AKI?

- Serum creatinine of 2.0-2.9 times baseline

- Urine output of <0.5ml/kg/hr for >12 hours

What is stage 3 AKI?

- Serum creatinine of 3.0 times baseline

- Urine output of 0.3ml/kg/hr for >24 hours or anuria for >12 hours

What is the treatment for high uric acid in the blood?

Allopurinol