investigations including imaging techniques especially ultrasonography (ultrasound),
magnetic resonance imaging (MRI), computerised tomography (CT) scanning, micro-
scopic examination of biopsy samples and liver function tests. Four enzymes are
routinely assayed to aid differential diagnosis:- Aspartate aminotransferase(AST)andalanine aminotransferase(ALT): As previously
stated, these enzymes are widely distributed but their ratios in serum are characteristic
of the specific cause of liver cell damage. For example, an AST/ALT ratio of less than 1
is found in acute viral hepatitis and fresh obstructive jaundice, a ratio of about 1 in
obstructive jaundice caused by viral hepatitis and a ratio of greater than 1 in cases of
cirrhosis. - g-Glutamyl transferase(GGT): This enzyme transfers ag-glutamyl group between
substrates and may be assayed by the use ofg-glutamyl-4-nitroaniline as substrate and
monitoring the release of 4-nitroaniline at 400 nm. GGT is widely distributed and is
abundant in liver, especially bile canaliculi, kidney, pancreas and prostate but these do
not present themselves by contributing to serum levels. Raised activities are found in
cirrhosis, secondary hepatic tumours and cholestasis and tend to parallel increases in
the activity of alkaline phosphatase especially in cholestasis. Its synthesis is induced by
alcohol and some drugs also cause its serum activity to rise. - Alkaline phosphatase(AP): This enzyme is found in most tissues but is especially
abundant in the bile canaliculi, kidney, bone and placenta. It may be assayed by using 4-
nitrophenylphosphate as substrate and monitoring the release of 4-nitrophenol at
400nm. Its activity is raised in obstructive jaundice and when measured in conjunction
with ALT can be used to distinguish between obstructive jaundice and hepatitis since its
activity is raised more than that of ALT in obstructive jaundice. Decreasing serum activity
of AP is valuable in confirming an end of cholestasis. Raised serum AP levels can also be
present in various bone diseases and during growth and pregnancy.
16.3.4 Kidney disease
The kidneys, together with the liver, are the major organs responsible for the removal of
waste material from the body. The kidneys also have other specific functions including
the control of electrolyte and water homeostasis, and the synthesis of erythropoietin.
Each of the two kidneys contains approximately 1 million nephrons that receive the
blood flowing to the kidneys. Blood flowing to the kidneys is first presented to the
glomerulus of each nephron which filters the plasma water to produce theultrafiltrateor
primary urineremoving all the contents of the plasma except proteins. Each nephron
produces approximately 100 mm^3 of primary urine per day giving a total production of
primary urine by the two kidneys of approximately 100–140 cm^3 per minute or 200 dm^3
per day in a healthy adult person. This is referred to as theglomerular filtration rate
(GFR). The primary urine then encounters the tubule of the nephron that is the site of the
reabsorption of water and the active and passive reabsorption of lipophilic compounds
and cellular nutrients such as sugars and amino acids and the active secretion of others.
These two processes in combination result inthe production of approximately 2 dm^3 of
urine per day that is collected in the urinary bladder.646 Principles of clinical biochemistry