Download INVESTIGATION OF RENAL AND URINARY TRACT DISEASE

INVESTIGATION OF RENAL AND
URINARY TRACT DISEASE
Tests of function
Glomerular filtration rate (GFR) GFR is the rate
at which fluid passes into nephrons after
filtration and measures renal excretory
function. GFR is proportionate to body size so
the reference range is usually expressed after
correction for body surface area as 120 ± 25
mL/min/1.73 m2.
Direct measurement of GFR by injecting and
measuring the clearance of compounds that
are completely filtered and not reabsorbed by
the nephron (inulin, radiolabelled
ethylenediamine-tetraacetic acid (EDTA)) is
inconvenient and is usually reserved for
special circumstances (e.g. for potential live
kidney donors).
Serum levels of endogenous compounds
excreted by the kidney give useful
information. Blood urea is not the best test, as
it increases with high protein intake (including
absorption of blood from the gut after a
gastrointestinal haemorrhage) and in catabolic
states, and is reduced in liver failure (low
production from protein) and anorexia or
malnutrition (low protein intake).
Serum creatinine reflects GFR more reliably than
urea, as it is produced from muscle at a
constant rate and almost completely filtered
at the glomerulus. If muscle mass remains
constant, changes in creatinine concentration
reflect changes in GFR . However, the
reference range for creatinine values is wide
because of variations in muscle mass; in
patients with low muscle mass (e.g. the
elderly) serum creatinine may not be above
the reference range until GFR is reduced by >
50%.
Measuring GFR
• Direct measurement using labelled EDTA or
inulin
• Creatinine clearance (CrCl)
– Minor tubular secretion of creatinine causes CrCl
to exaggerate GFR when renal function is poor,
and can be affected by drugs (e.g. trimethoprim,
cimetidine)
– Needs 24-hr urine collection (inconvenient and
often unreliable)
Estimating GFR with equations
• Cockcroft and Gault (C&G) equation
– Reasonably accurate at normal to moderately
impaired renal function
– Estimates CrCl, not GFR
– Requires patient weight
The Modification of Diet in Renal Disease
(MDRD) study equation
– Performs better than C&G at reduced GFR
– Requires knowledge of age and sex only
– Can be reported automatically by laboratories
Urinalysis
Examination of an aliquot of urine provides
important information on kidney function.
Dipsticks may be used to screen for blood and
protein semi-quantitatively . Urine microscopy
can detect red cells of glomerular origin and
red cell casts, indicative of intrinsic renal
disease. Flow cytometry can also be used to
screen for white blood cells and bacteria.
Crystals (e.g. of calcium oxalate, cysteine or
urate) may be seen in renal calculus disease,
although calcium oxalate and urate crystals
are also sometimes found in normal urine that
has been left to stand. Urine pH can provide
diagnostic information in the assessment of
renal tubular acidosis , and a persistently low
specific gravity may be found in diabetes
insipidus .
Timed (usually 24-hour) urine collections are
now used less often to measure GFR or
protein excretion , but are still required to
measure excretion rates of sodium and of
solutes that can form renal calculi such as
calcium, oxalate and urate . Other dynamic
tests of tubular function, including
concentrating ability , ability to excrete a
water load and ability to excrete acid , are
valuable in some circumstances.
Imaging techniques
Plain X-rays may show the renal outlines (if
perinephric fat and bowel gas shadows
permit), opaque calculi and calcification
within the renal tract.
Ultrasound
This quick, non-invasive technique is the first
and often the only method required for renal
imaging. It can show renal size and position,
detect dilatation of the collecting system ,
distinguish tumours and cysts, and show other
abdominal, pelvic and retroperitoneal
pathology.
Doppler techniques can show blood flow in
extrarenal and larger intrarenal vessels. The
resistivity index is the ratio of peak systolic
and diastolic velocities, and is influenced by
the resistance to flow through small intrarenal
arteries. It may be elevated in various
diseases, including acute glomerulonephritis
and rejection of a renal transplant
In addition, it can image the prostate and
bladder, and estimate completeness of
emptying in suspected bladder outflow
obstruction. In CKD ultrasonographic density
of the renal cortex is increased and
corticomedullary differentiation is lost.
However, renal ultrasound is operatordependent, and it is often less clear in obese
patients.
Intravenous urography (IVU)
While intravenous urography has been largely
replaced by ultrasound and/or CT urography
for routine renal imaging, the technique
provides excellent definition of the collecting
system and ureters, and remains superior to
ultrasound for examining renal papillae,
stones and urothelial malignancy .
The disadvantages of this technique are the
need for an injection, time requirement,
dependence on adequate renal function, and
exposure to irradiation and contrast medium
Renal arteriography and venography
The main indication for renal arteriography is to
investigate suspected renal artery stenosis or
haemorrhage. Therapeutic balloon dilatation
and stenting of the renal artery may be
undertaken, and bleeding vessels or
arteriovenous fistulae occluded.
Computed tomography (CT)
CT is particularly useful for characterising mass
lesions within the kidney , or combinations of
cysts with masses. It gives clear definition of
retroperitoneal anatomy regardless of obesity.
Even without contrast medium it is better
than IVU for demonstrating renal stones.
In CT urography, after a first scan without
contrast, scans are repeated during
nephrogram and excretory phases. This gives
more information but entails a substantially
larger radiation dose than IVU.
Magnetic resonance imaging (MRI)
• MRI offers excellent resolution and distinction
between different tissues . Magnetic
resonance angiography (MRA) uses
gadolinium-based contrast media, which may
carry risks for patients with very low GFR .
MRA can produce good images of main renal
vessels but may miss branch artery stenoses.
Radionuclide studies
These are functional studies requiring the
injection of gamma ray-emitting
radiopharmaceuticals which are taken up and
excreted by the kidney, a process which can be
monitored by an external gamma camera.
Renal biopsy
Indications
• Acute renal failure that is not adequately
explained
• CKD with normal-sized kidneys
• Nephrotic syndrome or glomerular proteinuria in
adults
• Nephrotic syndrome in children that has atypical
features or is not responding to treatment
• Isolated haematuria or proteinuria with renal
characteristics or associated abnormalities
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Contraindications
Disordered coagulation or thrombocytopenia.
Aspirin and other antiplatelet agents increase
bleeding risk
Uncontrolled hypertension
Kidneys < 60% predicted size
Solitary kidney (except transplants) (relative
contraindication)
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Complications
Pain, usually mild
Bleeding into urine, usually minor but may
produce clot colic and obstruction
Bleeding around the kidney, occasionally
massive and requiring angiography with
intervention, or surgery
Arteriovenous fistula, rarely significant
clinically
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