Page 230 - Medicine and Surgery

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226 Chapter 6: Genitourinary system

In the absence of proteinuria, it is important to ex- levels of glucose or myeloma chains in the urine, SG

clude cancer of the urinary tract and patients should is high even when the patient is not dehydrated.
be referred to a urologist. Urinary pH: In metabolic acidosis, urine pH falls to
below 5.3, unless the cause of metabolic acidosis is
Investigations and procedures RTA. Urinary tract infections with bacteria such as
Proteus, which produce urease, cause the urine pH to
rise to neutral or even alkaline levels.
Urine tests

Dipstick testing
Quantification of proteinuria
The basic test includes blood, protein, glucose, specific
This is done on patients who have persistent proteinuria.
gravity and urinary pH. More complex dipstick tests are
There are two main methods.
available which test for ketones (raised in starvation and
1 24-hour urine collection: Unfortunately this method
diabetic ketoacidosis), leucocyte esterase, which signifies
is inaccurate and often inconvenient for the patient.
white cells, and nitrites, which signifies the presence of
They must empty their bladder when they get up in
certain bacteria that convert nitrates (found in normal
themorning,thenbegincollectinginacontainerevery
urine) to nitrites.
single drop of urine passed for the next 24 hours, up
Blood: Dipstick testing for blood is much more

to and including the first emptying of the bladder the
sensitive than the naked eye. False positives occur
next morning. This is then sent to the laboratory for
with myoglobinuria and haemoglobinuria. Cross-
total protein quantification.
contamination may occur if females are menstruat-
2 Protein-to-creatinine ratio: This is performed on a
ing. If truly positive, it should be investigated as for
random urine specimen and is accurate, straightfor-
haematuria (see page 224).
ward and convenient. This test is simpler using con-
Protein: Normal protein excretion is <150mg/day,

ventional units as used in the USA, because urinary
of which albumin normally forms about 35 mg.
protein concentration and serum creatinine concen-
Dipstick for protein will be negative unless protein
tration are both expressed in mg/dL, so a ratio of 3.8
excretion is >300–500 mg/day. It mainly detects uri-
represents a 24-hour protein excretion of 3.8 g/1.73
nary albumin, and is insensitive for other proteins 2
m .In the United Kinddom, urinary creatinine is ex-
such as immunoglobulin light chains (Bence Jones
pressed in mmol/L so the result needs to be multiplied
protein) which require specific tests. Very dilute urine
by 0.088:
will underestimate the degree of proteinuria, and con-
centrated normal urine may show a trace of pro- 2
Protein excretion (g/1.73m )
tein. If 1+ or more of protein is detected, further
Urinary protein (mg/dL) × 0.088
quantification of protein should be done (see be- =
low). False positives can occur with radiocontrast Urine creatinine (mmol/L)
agents.
Glucose: Presence of glucose is almost always due to Microalbuminuria

diabetes mellitus. Glucose is not normally found in Patients with proteinuria, which is greater than normal
the urine until the plasma glucose concentration is butlessthandetectableondipstick,have‘microalbumin-
≥10 mmol/L. If found with lower levels, this suggests uria’. This is defined as albumin excretion of between 35
an inability of the kidney to reabsorb filtered glucose and 200 mg/24 hour. It is an early indicator of diabetic
due to dysfunction in the proximal tubule, such as kidneydisease,andisalsofoundinotherconditionssuch
occurs in multiple myeloma, renal tubular acidosis ascardiovasculardiseaseevenwithoutrenalimpairment.
(RTA) and pregnancy. There are now bedside testing kits for detecting microal-
Specific gravity (SG): This gives an estimation of buminuria (extra sensitive forms of dipsticks), but lab-

urine concentration, it is useful in acute renal failure, oratory testing is more accurate, using a random urine
investigation of polyuria or syndrome of inappropri- sample to compare urinary albumin-to-creatinine con-
ate anti-diuretic hormone (SIADH). If there are high centration (>3isabnormal).