Investigation of Renal Function 1
Clinical Biochemistry Lecture #6
(Fourth Year Medicine)
by
Prof. Dr. M.S.M. Ardawi
BMS(Oxford),MA(Oxford),PhD(Oxford), DSc(Oxford),FRCPath(Lond),MMEd(Amsterdam)
(Year 2009G)
• Why test renal function?
• Role of biochemical testing in renal function.
• Understand the definition of the Chronic renal insufficiencyin relation to:
√ Urea / Creatinine
√ Clearance studies
√ Na+ / K+
√ Acid-base
√ Ca++ / PO4-
√ Urate
• Harvest
Learning Objectives:
By the end of this lecture, the student will:
• Describe the role of biochemical testing in renal function.
Why Test Renal Function?
§ To identify renal dysfunction.
§ To diagnose renal disease.
§ To monitor renal disease progress.
§ To monitor response to treatment.
§ To assess changes in renal function that may impact on therapy (e.g. Digoxin, chemotherapy).
Case #1.
A 60-year old man presented to KAUH with wt. Loss, generalized weakness and lethargy of 8 months duration. On examination, the patient was found to be slightly anaemic and had a BP of 180/110 mmHg. His urine contained protein but no glucose. A blood sample was taken for analysis.
Serum Sodium 130 mmol/L
Potassium 5.2 mmol/L
HCO3 16 mmol/L
Urea 43 mmol/L
Creatinine 640 umol/L
Calcium 1.95 mmol/L
Phosphate 2.42 mmol/L
ALP 205 IU/L
Hb 9.1 g/dl
What is the diagnosis?
Renal failure
Acute
(hours-days) Chronic
(months-years)
Differentiation of acute and chronic renal failure
Acute Chronic
History Short (days-weeks) Long (months-years)
Haemoglobin concentration Normal Low
Renal size Normal Reduced
Renal osteodystrophy* Absent Present
Peripheral neuropathy† Absent Present
* Osteodystrophy is bone disease. † Peripheral neuropathy is disease or dysfunction of nerves supplying the limbs and peripheral tissues.
Definition of Chronic Kidney Disease
The clinical consequences of CRF
Signs and Symptoms of Renal Failure
§ Symptoms of Uraemia (nausea, vomiting ,lethargy).
§ Disorders of Micturation (frequency, nocturia, retention, dysuria).
§ Disorders of Urine volume (polyuria, oliguria, anuria).
§ Alterations in urine composition (haematuria, proteinuria, bacteriua, leukocyturia, calculi).
§ Pain.
§ Oedema (hypoalbuminaemia, salt and water retention).
Presentations of CRF
Asymptomatic serum biochemical abnormality Asymptomatic proteinuria/haematuria
Hypertension
Symptomatic primary disease Symptomatic uraemia
Complications of chronic renal failure
Renal insufficiency
Pre-renal
Renal
Post-renal
Stages of CRF
Stage of chronic
renal failure GFR*
(mL/min)
Symptoms of uraemia or its complications
Serum biological
derangements Comment Mid renal
impairment >75 None None Not clearly
progressive
Mild 50-75 None Subtle Early bone disease
commences
Moderate 25-50 Mild Mild Anaemia starts
Severe 10-25 Moderate Moderate Salt and water
retention evident End-stage <5-10 Severe Severe Dialysis or renal
transplantation necessary
• CRF is characterized by:
↑ [Urea] p
Depending on the Severity
of CRF
↑ [Creatinine] p
↑ [K+] p
↑ [PO4ˉ] p
↓ [Ca] p
↑ [Urate] p
↑ Anion gap metabolic acidosis
Sodium
• In uncomplicated, progressive CRF, Na-balance is maintained until ↓ GFR to very low levels (5ml/min)
↑ Urinary Na excretion per nephron
FENa (%) =
↑ Renal FENa up to 20% in CRF
<1% in healthy subjects
UNa Pcr
─── x ─── x 100 PNa Ucr
Potassium
• Patients with progressive CRF usually maintain K- balance until GFR → ↓ 20 ml/min
([Creatinine ]p = 0.30 – 0.35 mmol/L)
Due to the ability of the kidney to
↑ FEK by ~ 10-20x.
The biochemical course of typical patient with chronic renal failure.
Acid-base
• Normal renal handlings of H+ and HCO3- are maintained until GFR ↓ below 30ml/min
([Creatinine ] p = 0.30 mmol/L)
• Further ↓ GFR Retention of H+ and the [HCO3-]p progressively falls to level of 12-15 mmol/L
Acid-base
• As the GFR decreases, anions such as sulphates, phosphates, etc., will be retained in the blood, and along with the positive H+-balance
High Anion Gap metabolic acidosis
A feature of uncomplicated CRF which rarely ↑ above 30 mEq/L.
N.B. If >30 mEq/L (e.g. lactic acidosis, Ketoacidosis)
Calcium/Phosphate
• In CRF, there is a tendency towards a negative calcium balance
Hypocalcaemia
√ Is usually mild ([Ca]p ~ 1.80-2.00 mmol/L)
√ Occurs late in the disease
√ Is due to ↓ intake and ↑ [PO4-]p
• PO4 balance is maintained until GFR falls below 10-20 ml/min.
• Hyperphosphataemia results due to ↑ FEPO4 resulting from ↑ activity of PTH
How hypocalcaemia and secondary
hyperparathyroidism develop in renal disease
Progressive nephron destruction
Decreased formation of 1,25 (OH)2 D3
Decreased intestinal absorption of
Ca2+
Decreased serum Ca2+
Increased serum phosphate Increased phosphate
retention
Increased biosynthesis and secretion of PTH
Calcium/Phosphate
Thus, severe CRF may be associated with:
√ Hypocalcaemia
√ Hyperphosphataemia
√ ↑ [ALP]
√ Vitamin D deficiency
√ ↑ [PTH] p
Urate
• In uncomplicated CRF, [urate] p, begins to rise when the GFR falls below 20-30 ml/min.
Decreased excretion and increased production
• As failure progresses, [urate] p, rises further, usually plateauing ~ 0.6 mmol/L.
• Values > 0.6 mmol/L [urate] p,
Main consequences of CRF
Mechanism Example Consequence
Decreased excretion
Decreased biosynthesis
Altered metabolism
Uraemic toxins, including nitrogenous wastes
Salt and water Phosphate Acid
Potassium
Erythropoietin
Activation of vitamin D
Dyslipidaemia Sex hormones
Uraemic syndrome Volume overload, hypertension
Hyperparathyroidism, Metastatic calcification Metabolic acidosis
Hyperkalaemia Anaemia
Osteomalacia,
Hyperparathyroidism Atherogenesis
Abnormal reproductive function
Common causes of end-stage renal failure
* Percentage incidence
Glomerulonephritis Diabetes mellitus Hypertension
Polycystic kidney disease Vesicoureteric reflux Analgesic nephropathy Unknown
Other
30 25 10 5 5 5 10 10
What is the role of biochemical testing?
Presentation of patients ü Routine urinalysis
ü Symptom or physical sign
ü Systemic disease with known renal component.
Effective management of renal disease depends upon establishing definitive diagnosis
ü Detailed clinical history
ü Diagnostic imaging and biopsy.
Role of Biochemistry
ü Rarely establishes the cause ü Screening for damage
ü Monitoring progression of impairment.
Biochemical Tests of Renal Function
Urinalysis
- Appearance - Osmolality - pH
- Glucose - Protein - Urine
sediments
Other tests
- Urea
- Creatinine - Electrolytes - Acid base
balance - UA
GFR
- Clearance tests
Tubular function tests
The place of biochemical testing in urinalysis
Urinalysis 1
Urinalysis 2
• Fresh sample = valid sample
• Appearance
Blood Colour (Hb or Mb)
Turbidity
(Infection, Nephrotic syndrome)
• Specific gravity (SG) → sticks measure ionic species only (not glucose).
• pH
Normal = acidic (except after meal).
Red Urine
Clear Cloudy
Hemoglobinuria Myoglobinuria RBCs Present (Hematuria)
Red Plasma Clear Plasma
Differentiation of red urine testing chemically positive for blood
Urinalysis 3
[Urea]
pand [Creatinine]
p• Waste products
• Plasma levels usually indicate ↓ renal excretion rather ↑ production
• [Urea] p and diet
• Excretion rate is dependent on GFR
• GFR has to ↓ 40-60% before there is ↑ [Urea] p and [Creatinine] p in the absence of increased production
• Patients with renal failure may be classified on the basis of their [Creatinine]p:
1. Mild ( <0.20 mmol/L)
2. Moderate (0.20-0.40 mmol/L) 3. Severe (>0.40 mmol/L)
(Ref. range 0.06-0.12 mmol/L)
√ Production of creatinine and its [ ] p vary with muscle mass
√ Creatinine measurements (Jaffe’s methods is affected by acetoacetate and cefoxitin).
Decreased GFR
Prerenal: shock/haemorrhage dehydration
congestive cardiac failure
Renal: acute and chronic renal failure
Post-renal: obstructive lesions of the urinary tract Analytical interference
Ketosis: acetoacetate
Drugs: cephalosporins, e.g. cefoxitin
Causes of an Increased Plasma Creatinine Concentration
Difficulties
o Concentration depends on balance between input and output.
o Production determined by muscle mass which is related to age, sex and weight.
o High between subject variability but low within subject.
o Concentration inversely related to GFR.
- Small changes in creatinine within and
around the reference limits = large changes in GFR.
o Reference limits can be misleading
[Creatinine] p
Cystatin-C
• Cysteine proteinase inhibitor C (MW13000).
• Small size and freely filtered at glomerulus.
• Constant production rate by all nucleated cells.
• No known extra-renal excretion routes.
• Correlation of 1/[cystatin C] (r = 0.81) with Cr=EDTA clearance is better than 1/[Plasma Creatinine] (r=0.51)
• Not influenced by muscle mass, diet or subjects sex.
Cystatin C vs Serum Creatinine
Cystatin C more sensitive in
“creatinine blind” early GFR loss.
Cystatin C assay less precise.
Cystatin C more expensive.
Chronic Renal Failure: Prognosis
1
Creatinine
Time
Measurement of Glomerular Filtration Rate (GFR)
• GFR is essential to renal function.
• Most frequently performed test of renal function.
• Measurement is based on concept of clearance
“The determination of the volume of plasma from which a substance is removed by glomerular
filtration during it’s passage through the kidney”.
GFR using Inulin
• Golden Standard.
• Plant polysaccharide.
• Complex procedure
- Bolus dose followed by constant infusion.
- Timed urine samplings, with blood samples taken midpoint of collection periods, for inulin assay.
- GFR is taken as the mean for each period.
Isotopic GFR
• 99mTc-DTPA 51Cr-EDTA
• Single bolus injection with blood taken for isotopic counting at intervals.
• Extrapolation of Log plot of counts against time enables determination of initial volume of
distribution (V0).
• GFR = V0x(log e2)/t½
- t½ = half life for decrease in plasma radioactivity
Creatinine Clearance 1
§ Time urine collection for creatinine measurement (usually 24h)
§ Blood sample taken within the period of collection.
Problems
• Practical problems of accurate urine collection and volume measurement.
• Within subject variability = 11%, Critical Difference = 33%.
• Interference in creatinine measurement.
Cu Vol Creatinine clearance = ――― x ―――
Cp Time
Urinary [creat] (mmol/L)
Ref range: 90-120 ml/min (1.5-2.0 ml/s) Correlates with GFR
Creatinine Clearance 2
Volume of urine in ml
In seconds or minutes Plasma [creat]
(mmol/L)
The relationship between GFR and serum [creatinine].
Properties of Agents used to Determine GFR
Property Urea Creatinine Inulin 99mTcDTPA Not protein
bound Yes Yes Yes Yes
Freely filtered Yes Yes Yes Yes
No secretion or
absorption Flow related
reabsorption Some
secretion Yes Yes
Constant endogenous
production rate
No Yes No No
Easily assayed Yes Yes No No
Creatinine Clearance 3
§ Overestimates GFR (1.1 to 1.2 x Inulin Clearance).
GFR of 80-90 mL/min
o tubular secretion of creatinine (inhibited by cimetidine).
o true for transplanted kidneys.
o overestimates GFR greater in renal failure.
§ In children ratio is closer to 1.0 but as GFR falls the creatinine rises disproportionately and ratio of CC to inulin can = 2.0.
§ Time consuming, inconvenient and accurate?
Tests of Tubular Function
• Proximal Tubular Function o Phosphate reabsorption o Aminoaciduria
o Glycosuria
o Fractional HCO3 – excretion.
• Distal tubular function
o Acidification (ammonium chloride load).
o Concentration (water deprivation test).
• Why test renal function?
• Role of biochemical testing in renal function.
• Understand the definition of the Chronic renal insufficiencyin relation to:
√ Urea / Creatinine
√ Clearance studies
√ Na+ / K+
√ Acid-base
√ Ca++ / PO4-
√ Urate
Harvest
• Describe the role of biochemical testing in renal function.
