Measurement of renal function Flashcards
Why measure renal function
Identification of renal impairment in your patient
Modification of dosage of drugs which are cleared by the kidneys
Patients at risk of developing renal failure
Extremes of age: neonates and the elderly
Polypharmacy
Specific disease states: hypertension, chronic heart failure, diabetes, arthritis, renal disease, recurrent urinary tract infections
Patients receiving long term analgesics
Transplant patients
Drug therapy
Patients undergoing imaging procedures
Monitoring a patient’s renal function
Patients clinical condition
Modern imaging techniques
Biochemical data
Fluid balance affected
Clinical sign: oedema
Symptoms: breathlessness
Electrolyte refulation especially K+, Na+, PO42- and Ca2+ affected
Clinical signs: abnormal ECG, absent P waves, broad QRS complex, peaked T waves
No symptoms
EPO production affected
Clinical signs: pallor
Symptoms: fatigue
Vitamin D3 affected
Clinical signs: osteomalacia
Symptoms: bone pain
Excretion affected
Clinical signs: raised blood urea and creatinine concentrations
Symptoms: pruritis, nausea and vomiting
Acid base balance affected
Clinical signs: low blood pH and bicarbonate levels
Symptoms: deep and rapid respiration
Use of bedside clinical data
Weight charts
Fluid balance chart
Degree of oedema
Results of urine dipstick testing (urinalysis for protein, blood, glucose)
Modern imaging techniques
Macroscopic views of renal blood flow, filtration and excretory function
Some used clinically, some only currently used experimentally in the lab
Renography
Gamma camera planar scintigraphy
Positron emission tomography
Single photon emission computerised tomography
Biochemical data
Useful for identifying renal impairment
Blood markers of renal function
- plasma or serum creatinine
- plasma or serum urea or blood urea nitrogen
Creatinine
Breakdown product of creatine phosphate in muscle
Generally produced at a constant rate
Filtered at the glomerulus with some secretion into the proximal tubule
Normal range in plasma: 40-120umol/L
Plasma creatine increased by
Large muscle mass, dietary intake
Drugs which interfere with analysis
Drugs which inhibit tubular secretion
Ketoacidosis
Ethnicity (higher creatine kinase in black population)
Plasma creatinine decreased by
Reduced muscle mass
Cachexia/ starvation
Immobility
Pregnancy (increased plasma volume in mother)
Severe in liver disease
Plasma creatinine as an indicator of renal function
Normal adult adult range: 100 (40-120)
Normal kidney function: 120- 150
Mildly reduced kidney function: 150-300
Moderately reduced kidney function: 300-500
Severely reduced kidney function: 500-700
Very severely reduced kidney function: >700
Urea
Liver produces urea in urea cycle as waste product of protein digestion
Filtered at the glomerulus, secreted and reabsorbed in the tubule
Plasma urea BUN
Normal range: 2.5-7.5mmol/L
Blood urea nitrogen increased by
High protein diet
Hypercatabolic conditions
Gastrointestinal bleeding
Muscle injury
Drugs
Hypovolaemia
Blood urea nitrogen decreased by
Malnutrition
Liver disease
Sickle cell anaemia
SIADH
Examples of renal clearance
Filtered by the glomerulus and not reabsorbed e.g. inulin
Filtered and some of the filtered portion is reabsorbed e.g. Na+
Filtered and completely reabsorbed e.g. glucose and amino acids
Primarily secreted into the tubule e.g. PAH
Clearance
The volume of plasma completely cleared of a given substance in unit time
Provides information about functions of the kidney
- glomerular filtration
- tubular reabsorption
- tubular secretion
Renal clearance drawbacks
Measuring clearance means measurement of overall nephron function- 2 million nephrons in both kidneys
Gives sum of all transport processes occurring
No information about precise tubular sites of mechanisms of transport
Glomerular filtration rate
Rate at which filtrate is produced in the kidneys
Can be measured clinically and used as an indicator of renal function
Can be estimated by measurement of clearance of cretinine
More accurate estimation is measurement of inulin clearance as it is not secreted into tubule
Inulin clearance for measurement of GFR
Plant polysaccharide
Freely filtered but not secreted and not reabsorbed
Rate of excretion in urine equals rate of filtration by the kidneys
Drawbacks of inulin clearance to measure GFR
Not clinically useful
- must be administered by IV to get relatively constant plasma or serum levels
- chemical analysis of inulin in serum and urine is technically demanding
- could use radiolabelled compounds instead
- may also bind to proteins and distort results
- problems of IV infusion of GFR markers are avoided by using an endogenous substance with inulin like properties e.g. creatinine
Creatinine clearance
Filtered at the glomerulus but some also secreted into the proximal tubule
Using the equation would over estimate GFR by about 20% in humans
Colorimetry methods used to measure creatinine under estimates creatinine concentrations by about 20%
2 errors cancel each other out
Adjustments due to body surface area
Creatinine clearance can also be adjusted to take account of body surface area
Corrected creatinine clearance= meausre CrCl x1.71/BSA
Estimation of GFR using serum creatinine only
Allows estimation of GFR without having to collect urine samples
Cockcroft- Gault Formula
Modification of diet in renal disease formula
Using PAH clearance to measure renal blood flow
If a substance is completely cleared from the plasma its clearance rate will be equal to renal plasma flow
Clearance of PAH can be used to estimate this
Not normally present in blood
