S147202992100120X

NEPHROLOGY
Laboratory tests of renal
function
Learning objectives
After reading this article, you should be able to:
C
list six laboratory tests that assess renal function
C
calculate glomerular filtration rate
C
state normal blood and urine biochemistry values
Navid Wani
Tina Pasha
Abstract
The kidneys are vital organs in the management of fluid balance, waste
product removal, electrolyte homeostasis, acidebase balance and
endocrine function. Waste products removed by the kidney are urea,
uric acid and creatinine; other foreign products with similar physiochemical properties are also excreted. Urea and uric acid are by products of protein metabolism and creatinine is generated by the
metabolism of creatine compounds from muscle. The kidney regulates
fluid and electrolyte balance through controlling the composition and
volume of urine. In the proximal convoluted tubule and the loop of
Henle, 90% of sodium, potassium, calcium and magnesium are reabsorbed. Acidebase balance is achieved by regulating the excretion of
hydrogen ions and bicarbonate buffering. The kidney also has a number of endocrine functions including the production of renin and erythropoietin as well as hydroxylation of vitamin D. The kidneys receive
25% of cardiac output, generating 170e200 litres of ultrafiltrate per
day. Urine output is approximately 1.5 litres per day, which is concentrated ultrafiltrate through selective reabsorption of solutes and water.
In this article we will discuss tests frequently used to assess renal
function.
measured should be freely filtered and not reabsorbed, secreted
or metabolized along the path of the nephron, otherwise this will
lead to an inaccurate calculation of GFR.
The equation used for the calculation of GFR is:
Cs ¼ (Us V)/Ps
Where Cs is the volume of plasma cleared of the substance per
minute, Us is the urinary concentration of the substance, V is the
volume of urine produced per minute and Ps is the plasma
concentration of the substance.
A number of exogenous substances have been used to
calculate GFR. Such as 51creatinine EDTA inulin, 124iothalamate
and cystatin C.1 Radioisotopes are not commonly used in clinical
practice, as there are issues related to the safe disposal of the
isotope and the prolonged clearance from patients with chronic
renal disease.
Creatinine clearance
Given the practical issues discussed above, creatinine is the most
commonly used endogenous marker for assessing renal function.
Formed from the breakdown of creatine and phosphocreatine
from skeletal muscle, metabolism occurs at a relatively constant
rate. Creatinine is freely filtered at the glomerulus and not reabsorbed; however, it is actively secreted by the proximal convoluted tubule, which can be up to 15%. This may lead to over
estimation in creatinine clearance. However, there is some
attenuation by the plasma concentration also being overestimated. Secretion at the proximal convoluted tubule can be
abolished by cimetidine to get a true reading of GFR.2
Creatinine clearance is calculated from a 24ehour urinary
collection and serum creatinine sample taken during the time of
collection. Serum creatinine concentration is assumed to be stable during the 24-hour period. Creatinine clearance has also been
performed over shorter periods in catheterized patients.
Problems related to performing and measuring creatinine
clearance include time taken, inconvenience to the patient, errors
in urine collection and extra renal degradation of creatinine.
Keywords Acute kidney injury; biomarkers; creatinine; creatinine
clearance; glomerular filtration rate; renal function; urea; urinalysis
Royal College of Anaesthetists CPD Matrix: 1A01, 1A03, 2A03
Assessment of renal function
Assessment of renal function is performed through history and
examination in conjunction with the tests shown in Table 1 to
ensure a complete clinical picture is formed. The findings may
help to diagnose a renal or systemic disorder.
Glomerular filtration rate (GFR)
This is the rate that plasma is cleared of substances by filtration
of blood through the glomerulus into the Bowman’s capsule.
Measuring the clearance of specific substances is a surrogate of
GFR, which in turn provides an overall index of renal function.
Renal clearance is the volume of plasma cleared of an ideal
substance per unit time, measured in ml/min. The ideal particle
Commonly used tests of renal function
Urea
Creatinine
Creatinine clearance
Urine microscopy
Urine osmolarity
Navid Wani MBBS MD is a Specialist Trainee in Anaesthesia at
Manchester University NHS Foundation Trust, Manchester, UK.
Conflicts of interest: none declared.
Tina Pasha MBChB MRCP FRCA is a Consultant Anaesthetist at
Manchester University NHS Foundation Trust, Manchester, UK.
Conflicts of interest: none declared.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 22:7
Full blood count
Naþ
Kþ
Ca2þ and PO4Parathyroid hormone
Table 1
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NEPHROLOGY
overestimates GFR in obese or oedematous individuals and does
not account for ethnicity.
The MDRD Study Group developed an alternative to this
formula, indexed to body surface area. In its original form, it
uses six measurements to estimate GFR (eGFR), including blood
urea nitrogen (BUN) and albumin levels. A basic four-variable
form of the calculation containing serum creatinine, race, age
and gender is:
Creatinine-based equations of GFR
A number of formulae have been developed to calculate creatinine
clearance. However, all have limitations as they do not take into
account the variables that affect serum creatinine levels, which
include age, height, weight, gender, race, exercise and diet.
The two most commonly used formulae are Cockcroft-Gault
equation (CG) and Modification of Diet in Renal Disease study
group (MDRD) (Table 2).
The CG model estimates creatinine clearance (eCcr), and
hence GFR, based on serum creatinine, age, sex and body mass.
The original formula used weight in kilograms and creatinine in
milligrams per decilitre, as is standard in the USA:
eGFR (ml/minute/1.73m2) ¼ 186 (serum creatinine((mmol/
litre)/88.4) 1.154 age 0.203 1.12(if black) 0.742 (if female)
However, the MDRD estimate is a poor measure of GFR in healthy
individuals without renal pathology.3 For initial diagnosis of
chronic kidney disease the CKD-EPI formula is recommended by
NICE as the most appropriate method to estimate GFR; it is based
on the four-variable form of the MDRD equation but is more accurate, especially at higher GFR.4 These equations are not validated in children, in whom an alternative, the Schwartz equation
(see Table 2), should be used. Height in centimetres is multiplied
by an age-dependent constant; this total is then divided by the
serum creatinine concentration to give an estimation of GFR
indexed to body surface area. Creatinine-based equations have
many limitations, reflecting the variability of creatinine production
with many factors. Diuretics, spironolactone and triamterene, as
eCcr ¼ ((140 eage) weight (kg) (0.85 if female)) /72 X
serum creatinine (mg/dl)
Serum creatinine in the UK is measured in micromoles per litre,
the formula is modified and a constant is used for both men and
women to complete the estimation:
eCcr ¼((140-age) weight(kg) constant(m/f))/serum creatinine (mmol/litre).
The constant is 1.23 for men and 1.04 for women. This formula is
well supported as it provides a simple way to estimate GFR. It
Equations for assessment of renal function
1 The Cockroft-Gault equation (UK)
eCcr ¼
ð140 ageÞ weightðkgÞ ðconstantÞ
serum creatinine:ðmmol=litreÞ:
eCcr ¼ estimated creatine clearance
The constant is 1.23 for men and 1.04 for women.
This formula provides a simple way to estimate GFR.
2 The Modification of Diet in Renal Disease Study Group equation
eGFR ml/minute/1.73m2 ¼ 186 (serum creatinine(mmol/l)/88.4) 1.154 age
L0.203
1.12(if black) 0.742 (if female)
3 CKD EPI equation (for estimating GFR expressed for specified race, sex and serum creatinine in mg/dl)
GFR. 141_min(Scr/k, 1)a_max(Scr/k, 1)_1.209_ 0.993Age_ 1.018(if female) _ 1.159 (if black)
where:
Scr is serum creatinine in mg/dl,
k is 0.7 for females and 0.9 for males,
a is _0.329 for females and _0.411 for males,
min indicates the minimum of Scr/k or 1, and
max indicates the maximum of Scr/k or 1.
4 The Schwartz equation
eCcr.(ml/minute/1.73 m2 ¼ length in cm_k
serum creatinine.mg ¼ dl.
k ¼ 0.33 for prem infants
k ¼ 0.45 for infants term to 1 year
k ¼ 0.55 for children 1 year to 13 years
k ¼ 0.70 in adolescent males (females constant remains at 0.55)
5 Free water clearance (FWC)
FWC . V(1_Uosm/Posm)
V ¼ urine volume
Uosm. ¼ urine osmolality
Posm. ¼ plasma osmolality
Table 2
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NEPHROLOGY
well as trimethoprim, cimetidine and probenecid, interfere with
tubular secretion of creatinine, and can increase serum creatinine
concentrations while not reflecting alterations in GFR. Extremes of
muscle mass or breakdown, pregnancy, very low body mass
index, creatine supplements or rapidly changing renal function
impair accuracy and extrapolation of GFR.
All of these equations are based on an assumption of
stable creatinine production and excretion and were designed
to reflect chronic kidney disease. These equations are therefore
inappropriate for the assessment of acute kidney injury (AKI)
Studies have suggested that serum levels of cystatin are a better
indicator of GFR than serum creatinine.5,6 Cystatin C levels are less
dependent on age, gender, ethnicity, diet and muscle mass
compared to creatinine.7 It has been incorporated into the combined
creatinine-cystatin KDIGO (Kidney Disease Improving Global Outcomes) CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) equation to measure estimated GFR. Cystatin C levels may be
altered with smoking, thyroid disease and glucocorticoid therapy.
Creatinine and acute kidney injury
Serum creatinine remains the key player in assessing AKI. Creatinine levels may not rise until up to 60% of renal function is already
lost resulting in overestimation of GFR. This is the result of the
influence of extra renal factors explained above. Any rise in baseline creatinine even within the normal range (Table 3) should raise
suspicion of a significant decrease in renal function (Figure 1).4
NGAL
Neutrophil gelatinase-associated lipocalin (NGAL) is a protein of
the lipocalcin family, produced by the thick ascending loop of
Henle and the collecting duct during ischaemic injury to the
kidney.8 A meta-analysis of data from 19 studies has shown that
NGAL was a useful predictor of AKI with prognostic value; uses
included predicting the need for RRT, in-hospital mortality and
risk of contrast-induced nephropathy. NGAL has been assessed
in both the emergency and critical care settings as an indicator of
AKI. Rises in both plasma and urinary NGAL concentration has
been shown to occur earlier and greater than the rise in serum
creatinine concentration, therefore predicting development of
AKI.8 When NGAL was measured in post surgical patients it only
rose in those patients who had acute tubular necrosis (ATN).
NGAL has also been shown to rise in the presence of congestive
heart failure and vascular disease so may be less useful in those
with multiple comorbidities than healthy individuals with AKI.
Biomarkers of renal function
Serum urea
A product of protein metabolism, serum urea is freely filtered at the
glomerulus. Unfortunately it is not a reliable assessment of renal
function as serum urea concentrations are affected by the hydration
state of the patient, dietary intake of protein and liver function.
Cystatin C
Cystatin C is a low molecular weight protein produced by all the
nucleated cells in the body. It is produced at a constant rate and
removed by filtration in the glomerulus. Serum levels of cystatin C
are inversely proportional to GFR. Unlike creatinine, cystatin C is
reabsorbed and metabolized by the proximal convoluted tubules.
Interleukin 18
Interleukin 18 is produced by the proximal convoluted tubule of
the kidney and its production rises in parenchymal injury. Its
concentration is raised in urinary samples and peaks at 6 hours
post injury in the presence of acute kidney injury.9
Normal values
Blood biochemistry
(mmol/l)
Urine biochemistry
(mmol/l/24 hours)
Sodium 132e144
Potassium 3.5e5.5
Urea 3.5e7.4
Creatinine(male) 62e106
Creatinine(female) 44e80
Sodium 100e200
Potassium 30e90
Relationship of serum creatinine with glomerular
filtration rate (GFR)
160
140
Creatinine 9e17
Protein <0.15 g/24
hours
120
Venous bicarbonate 24e30
Chloride 95e110
Creatinine clearance
80e140 ml/minute
Osmolarity 275e295 mmol/kg
100
80
60
Osmolarity 50e1200
mosmol/kg
40
Osmolar gap (calculated measured
osmolarity) <10 mmol/litre
Calculated osmolality. 2 (Na þ
K)þglucose.þurea
Anion gap (Na þ Kþ-(Cl- þHCO3)
¼ 4-12 mmol/l
20
0
0
2
3
4
5
6
Serum creatinine, mg/dl
7
8
From Cirillo M. 2010. J Nephrol
Table 3
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1
Figure 1
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NEPHROLOGY
the concentration of the urine, providing an indication of tubular
function. Specific gravity can be altered under a number of circumstances, including glycosuria, proteinuria, drugs and extremes of age. Microscopic analysis of urine can identify the
presence of red blood cells (RBCs), white blood cells (WBCs),
epithelial cells, hyaline casts, RBC casts, WBC casts and bacteria.
Although non-specific, these findings can indicate significant
renal pathology. The presence of dysmorphic RBCs and, in
particular, RBC casts suggests glomerulonephritis or significant
tubule damage with RBC leakage. WBC casts may indicate
intrarenal disease such as pyelonephritis or glomerulonephritis
because these casts are formed in the kidney not in the distal
urinary tract. Hyaline casts are non-specific and can be found in
healthy patients. Bacteria are commonly seen in urine under
microscopy, often reflecting contamination. The diagnosis of
infection requires culture; however, a colony count of greater
than 100,000/ml from a true clean catch, catheter or suprapubic
sample carries greater significance. Microalbuminuria, as defined
by an albumin excretion of more than 30e300 mg/day, suggests
hypertensive disease in patients with diabetes. The presence of
macroalbuminuria (>300 mg/day) suggests diabetic nephropathy, which can be confirmed by the coexistence of a declining
GFR and elevated blood pressure.
Liver type fatty acid binding protein (L-FABP)
A protein found in the proximal convoluted tubule, measured in the
urine, L-FABP is produced at higher concentrations when kidney
parenchyma is exposed to hypoxia as shown in patients who underwent cardiac surgery. L-FABP has been found to be good at
predicting AKI, and the course of the disease over the next 7 days.10
Urinary angiotensinogen
Has been used to assess progression of AKI. Urinary angiotensinogen has been shown to be the most sensitive of the urinary biomarkers.11 Further studies are required to establish the
its true role in AKI.
Urinary microRNA
A heterogeneous group, many microRNAs have been identified
in patients in ICU with AKI. It has been shown to rise several
days before serum creatinine in patients with AKI.12 So far it has
not been established which urinary micro RNA has the best
sensitivity and specificity.
Tests for tubular dysfunction
Proximal tubular failure causes acidosis accompanied by a fall in
serum potassium, phosphate, urate and bicarbonate levels,
reflecting a failure of reabsorption. Usually, urea and creatinine
levels are normal. Acidification of the urine by oral ammonium
chloride is occasionally performed to determine the presence of
renal tubular acidosis. This measures the ability of the kidney to
produce acidic urine (pH < 5.3) in response to the ammonium
chloride. Ammonium chloride is metabolized by hepatocytes to
urea with the consumption of bicarbonate. Exclusions to this test
include alkaline urine in the presence of metabolic acidosis (diagnostic of renal tubular acidosis) and hepatic impairment.
Difficulties in interpretation can occur when urinary infection,
low potassium or elevated calcium are present.
A water deprivation test can be used to assess the ability of the
distal tubules to reabsorb water. If diabetes insipidus is suspected
then a water deprivation test is used.
Urinary osmolarity and body weight are repeatedly measured
while allowing only dry food for 8 hours. The measurement of urine
osmolarity provides information on the ability of the kidneys to
concentrate urine in response to water balance. The urine osmolarity and volume and body weight are measured at hourly intervals
until steady state or a urine osmolarity of 750 mosmol/litre is
reached. If the urine osmolarity fails to rise by 30 mosmol/litre, a
desmopressin test can be conducted. Recovery of concentrating
ability, which is indicated by an increase in the urine osmolarity of
greater than 50% after administration of desmopressin, suggests
cranial diabetes insipidus rather than a nephrogenic cause.
Urine tests for differentiation between pre-renal and
intrarenal failure
Pre-renal failure is characterized by concentrated urine with low
sodium excretion (<20 mEq/litre) and a fractional excretion of
sodium (FENa) of less than 1%. Intrarenal failure typically has a
FENa of greater than 2% and a urinary sodium concentration
over 20 mEq/litre. A raised ratio of urine to serum urea and
creatinine and a urinary osmolarity greater than 500 mosmol/
litre suggests that renal concentrating function is intact. This
supports a diagnosis of pre-renal failure. In practice, to facilitate
diagnosis results should be looked at in conjunction with history,
examination and serum results.
Endocrine tests in renal disease
Chronic renal impairment is associated with renal endocrine
insufficiency. Anaemia may suggest impaired erythropoietin
production in CKD. Elevated PTH is a non-specific test which
may indicate secondary or tertiary hyperparathyroidism reflecting alterations in 1,25-vitamin D3 production and calcium and
phosphate homeostasis.
A
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