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230 Chapter 6: Genitourinary system

Cystoscopy Serum creatinine: Creatinine is produced as a waste
product when creatine phosphate is broken down in
Arigid or flexible fibreoptic cystoscope is introduced
muscle. The amount produced is lower in those with
through the urethra in order to visualise the interior
low muscle bulk, in women, children and the elderly.
of the bladder. Flexible cystoscopy can be done under
It is freely filtered, a small amount is also secreted at
local anaesthetic, as a daycase procedure, but rigid cys-
the tubules. Plasma creatinine is increased by strenu-
toscopyisperformedunderanepiduralorgeneralanaes-
ous exercise, ingestion of meat, certain drugs (trimetho-
thetic. The bladder is distended with distilled water or
prim and cimetidine) impair tubular secretion. It is de-
saline, and forceps or diathermy loops can be inserted
creased in malnutrition, wasting diseases, immediately
through the instrument to take biopsies, and treat su-
after surgery and by corticosteroids. For these reasons
perficial bladder cancer (transurethral removal of blad-
there is wide variation in normal creatinine levels be-
der tumour – TURBT). The ureteric orifices can be in-
tween individuals. In most patients, serial or previous
spected, and fibreoptic ureteroscopes can be passed up,
measurements of creatinine are useful to monitor the
to look for ureteric lesions such as stones or carcinoma.
progress of renal function.
In addition, the ureteric orifice can be cannulated us-
Glomerular filtration rate (GFR): To assess the GFR
ing a fine-bore catheter, so that retrograde pyelography
the rate at which a substance is cleared from the plasma is
can be performed. Prophylactic antibiotics are needed, 2
measured. The normal GFR = 80–130 mL/min/1.73 m
to reduce the risk of a urinary tract infection.
body surface area. Clearance is defined as the ‘virtual’
volume of blood cleared (by the kidney) of solute per
unit time. If a substance is completely filtered by the
Measuring renal function
glomeruli and not secreted, absorbed or metabolised by
Renal function testing involves measuring urea and cre- the renal tubules then its urinary clearance equals GFR.
atinine and glomerular filtration rate (GFR). Urea and Creatinine almost fulfils these criteria, and is used in
creatinine may remain normal until more than half of clinical practice to measure GFR using a 24-hour urine
renal function is lost. collection:
Normalkidneysreceive25%ofthecardiacoutput.The
UV
volume that is ultrafiltrated per minute is the GFR, and it Urinary clearance = (usually expressed in mL/min)
P
is the sum of the filtration that occurs in all the function-
ing nephrons. When nephrons are lost or are not func- where U = urinary concentration, V = urine flow rate
tioning properly, there is compensation by the remain- and P = plasma creatinine.
ing nephrons by hyperfiltration, and improved solute Blood creatinine levels are inversely related to clear-
clearance. If there is poor blood supply to the kidneys, ance. This means that with normal renal function a small
due for example to hypotension or cardiac failure, the rise in creatinine means a large fall in GFR. Conversely
GFR will fall – even though the nephrons are intact. in patients with moderate to severe renal failure, i.e. a
Serum urea: Urea is freely filtered at the glomeruli, but low GFR, even a small further fall in GFR will result in a
variably reabsorbed by the tubules, and its production large increase in creatinine (see Fig. 6.2).
fluctuates considerably, even within an individual. It is 24-hour urinary collections are inconvenient and in-
higher following protein intake, in a catabolic state, af- accurate.Thecreatinineclearancecanbecalculatedfrom
tersteroids or gastrointestinal (GI) haemorrhage, and a patient’s serum creatinine using formulae correcting
lower when patients are not eating, and in liver dis- for factors like the patient’s age, sex and weight (which
ease.Indehydration,urearisesproportionallymorethan adjust for muscle mass). The best known of these is the
creatinine because it is avidly reabsorbed at the proximal Cockcroft and Gault formula:
tubules in a fluid-depleted state.
1.23 × (140 − age) × weight (kg)
Aureaabove normal therefore suggests renal fail- CrCl mL min =
Cr (µmol/L)
ure, GI bleeding or dehydration. If the creatinine
is also proportionally raised (creatinine is normally Forwomen multiply by 1.04 rather than 1.23. When
∼20× urea) above normal, this indicates intrinsic renal accurate GFR measurement is needed the rate of clear-
failure. ance of a radioisotope is used. This is indicated in severe