Renal Function Tests

Functions of Kidney

The kidney has five major functions

Ø      Removal of waste products/ toxic substances

Ø      Regulation of water and electrolyte balance

Ø      Regulation of acid-base homeostasis

Ø      Conservation of Protein

Ø      Endocrine functions

n      Erythropoietin production

n      Renin – angiotensin production

n      Calcitriol- Vitamin D metabolism

Why Test Renal Function?

n      To identify renal dysfunction.

n      To diagnose renal disease.

n      To reveal the principle site of pathology

n      To assess the degree of functional damage

n      To monitor disease progress.

n      To monitor response to treatment.

Clinical history

Ø      History of renal disease

Ø      Evidence of systemic illness

Ø      Family history of renal disease

Ø      Medications being taken

Signs and Symptoms of Renal Diseases

n      Symptoms of Uraemia (nausea, vomiting, lethargy)

n      Disorders of Urine volume (polyuria,  anuria)

n      Alterations in urine composition (haematuria, proteinuria)

n      Pain ( calculi)

n      Oedema (hypoalbuminaemia, water retention)

n      Disorders of Micturation (frequency, nocturia, retention, dysuria)

Biochemical Tests of Renal Function

n      Urinalysis

n      Appearance

n      Specific gravity and osmolality

n      pH

n      Glucose

n      Protein

n      Urinary sediments

n      Measurement of GFR

n      Creatinine clearance tests

n      Serum Creatinine/Urea

n      Cystatin C

n      Tubular function tests

Urine Routine Examination

n      Appearance

n      Urine volume (750-2500 ml/24h)

n      Anuria: 100 ml/24 h

n      Oliguria: 400 ml/24 h

n      Polyuria: >3000 ml/24 h

n      Specific  gravity (1005-1020)

n      Increased: Pre renal uraemia

n      Decreased: Diabetes insipidus

n      Fixed specific gravity e.g. CRF (1010)

n      pH (5.5-8.0)  – reflects acid base status

n      Protein

n      Glucose

n      Formed elements

n      Casts

n      Crystals

24 hour urine protein excretion

v     > 150mg/24 hour = proteinuria

v     Gives a more accurate assessment of the severity of the proteinuria

v     > 3.5 g/24 hour (with associated features) = nephrotic syndrome

Classification of Proteinuria

n      Tubular proteinuria

n      Tubular dysfunction

n      Overflow proteinuria

n      Glomerular proteinuria

n      Selective proteinuria

n      Non-selective proteinuria

n      microalbuminuria

Urine microscopy

n      To look for casts, white cells and red cells may give a clue to the diagnosis of:

n      glomerulonephritis

n      pyelonephritis

n      tubular damage

Serum Creatinine

n      Reliable test for glomerular function

n      Production determined by muscle mass which is related to age, sex and weight.

n      Reference range- 70-133  umol/l

n      Mild renal damage: <200 umol/l

n      Moderate renal damage: 200-400  umol/l

n      Severe renal damage: > 400  umol/l

Creatinine Clearance

n      Most sensitive test for renal function

n      The volume of blood from which creatinine is completely removed per unit time

n      Clearance = (U x V)/P

n      U  is urinary concentration of creatinine

n      V  is the rate of urine formation (mL/min)

n      P is the plasma concentration of creatinine

n      Units = volume/unit time (mL/min)

n      Cockcoft – Gault Formula*

CC = k[(140-Age) x weight (Kg))] / Creatinine (µmol/L)

k = 0.81 for males  &  0.85 for females

n      Creatinine clearance   =  U x V

______           =    mL / minute

P x 24 x 60

Indications

1. 1. Assessment of potential kidney donor
   2. Investigation of patient with abnormal renal function
   3. Patient on toxic drugs,which are excreted by kidneys

Precautions

1. 1. Take 3-4 L container
   2. Void all the urine in the container
   3. Avoid red-meat

Urea

n      Urea is synthesized in liver as a by –product of deamination of amino acids – Urea cycle

n      Serum urea concentration is used for assessment of renal function

n      Reference Range 2.5-6.7 mmol /L

Plasma Electrolytes

n      Plasma Sodium     (135-148 mmol/l)

n      Early CRF                  – Normal

n      ARF /Late CRF         –  Hyponatraemia

n      Plasma potassium    (3.5-5.0 mmol/l)

n      Early CRF         :      Normokalaemia

n      ARF /Late CRF :      Hyperkalaemia

n      Plasma Bicarbonate  (22-28 mmol/l)

n      ARF / CRF :   Low due to metabolic acidosis

n      Serum chloride (95-107 mmol/l)

n      Early CRF: Increased

n      Anion gap

n      (Na +K) – (CI + HCO3) = upto 16 mmol/l

n      Normal anion gap: early CRF

n      High anion gap: Late CRF

n      Serum calcium (2.05-2.55mmol/l)

n      Reduced in RF

n      Serum phosphate (0.8-1.45mmol/l)

n      Increased in RF

n      Serum Magnesium ( 1mmol/l)

n      Increased in RF

ARF : Prerenal : Renal

n      Urine sodium (spot)

n      Helpful in D/D of oliguria

n      PRU: Urine sodium < 20 mmol/l

n      ATN: Urine sodium > 40 mmol/l

n      Renal fractional  excretion of sodium

n      FE Na < 1%: PRU

n      FE Na > 1%: ATN, Obstructive                                                       nephropathy, CRF

n      Urine : plasma osmolality

n      PRU   > 1.5 : 1

n      ATN   < 1.1 : 1

n      CRF   1 : 1

Other tests

n      Blood Complete and E.S.R

n      Anaemia

n      Abnormal T.L.C and D.L.C

n      Plasma and urine electrophoresis if suspect multiple myeloma with Bence Jones proteinuria

n      Urine for myoglobin / haemoglobin

n      Renal biopsy if suspect glomerular disease

n      HbA1c to assess diabetic control

n      Renal ultrasound if suspect renal disease

n      Plain and contrast X-ray

n      C.T and M.R.I