Introduction and chemical pathology lab

Question 1

The Department of Chemical Pathology processes samples and measures the concentrations of many important metabolites. Changes in the concentration of metabolites can suggest particular illnesses.

What are the some of the most commonly requested tests?

Answer 1

• Liver function tests (LFTs)
• Urea and electrolytes (U&Es) to include Na, K, urea and creatine
• Calcium and phosphate.
• Hormone assays (commonly thyroxine, TSH and cortisol)
• Glucose

Question 2

Red, yellow, purple and grey top blood tubes have what inside them?

Answer 2

Red top - have none

Yellow top - have gel to speed up clot

Purple top - have potassium EDTA (anticoagulant)

Grey top - have fluoride/oxalate

Question 3

What type of sample is collected from the tube (serum or plasma) and which colour tube top is required for:

• U&E
• Glucose
• HBA1c
• TFT
• Liver function tests:

Answer 3

U&E: serum in yellow/red top

Glucose: plasma in grey top

HBA1c: plasma in purple top

TFT: serum in yellow/red top

Liver function tests: in yellow/red top

Question 4

What’s the difference between serum and plasma? How does the colour of the tube top dictate which you get?

Answer 4

In the yellow/red top, the blood clots hence using up all clotting factors. Upon centrifugation, the clotted blood will be at the bottom, gel (of intermediate density) in the middle, and serum at the top.
=> Serum is free of any clotting factors.

However, in the presence of any anticoagulant (e.g. EDTA or heparin), the clotting factors are unused. Upon centrifugation, the blood separated so that red cells (not clotted) + neutrophils are at the bottom and plasma is at the top.
=> Plasma contains clotting factors.

Question 5

Describe the importance of the fluoride/oxalate in the grey top tubes for measuring blood glucose

Answer 5

Fluoride/oxalate stops glycolysis (poisons red cells) so glucose levels stabilise.

Otherwise…
Live red blood cells would consume the glucose (lowering the glucose measurement)

Question 6

What can skew the electrolyte (potassium) measurements of the serum?

Answer 6

Haemolysis – red cells contain a lot of potassium so haemolysis caused by poor collection will result in an increased serum potassium concentration

Question 7

What does the blue top mean?

Answer 7

• Contains citrate (anticoagulant)
• Used to measure clotting factors
• Citrate binds to calcium to prevent clotting (reversibly)
• When you get a sample of blood in a citrate bottle, you add just the right amount of calcium to trigger the clotting cascade
• By measuring the time taken to clot, you can tell if there’s enough clotting factors

Question 8

What do green top tubes contain?

Answer 8

Lithium

Question 9

When do you need to actually contact a chemical pathologist yourself?

Answer 9

• When you want the sample to be rapidly centrifuged out of hours
• When you want to measure labile hormones such as insulin (the process of clotting destroys the peptide, so measurement must be quick)
• When you urgently need CSF glucose and protein to be measured

Question 10

What is creatinine a marker of?

Answer 10

Glomerular filtration rate

If it is normal, then the GFR is normal. Very little is absorbed or secreted by the tubules

Question 11

What is urea a marker of?

Answer 11

Dehydration

Question 12

How does urea and creatine levels change in renal failure?

Answer 12

Their levels increase

Question 13

What does damage to the liver commonly result in?

Answer 13

• Only very small amounts of liver enzymes should enter the bloodstream.
• Damage to the liver may result in extra amounts of these enzymes leaking into the blood.
• Particular diseases seem to be associated with particular patterns of liver enzymes leaking.

Question 14

Name four liver enzymes commonly measured

Answer 14

1. Alkaline phosphatase
2. Aspartate amino-transferase (AST) - measure in jaundice
3. Alanine amino-transferase (ALT)
4. Gamma-glutamyl transferase (GGT) - measure in jaundice

Question 15

How does albumin change in liver disease?

Answer 15

Albumin is made in the liver and so if the liver fails, then albumin levels decrease.

Question 16

Why is bilirubin made in the liver and where does it go?

Answer 16

The liver breaks down haemoglobin in old red blood cells into bilirubin and deposits in the biliary tree and gallbladder for excretion via the GI tract. Bilirubin is dark yellow/brown (hence the colour of stools)
Blockage in the biliary tree => pattern of pale stools and dark urine; and increased bilirubin in blood => jaundice

Question 17

Where else, other than hepatocytes, is alkaline phosphatase produced?

Answer 17

pancreas

Question 18

Increased ALT is indicative of what?

Answer 18

Inflammation of liver tissue - viral hepatitis (so think, has the patient been abroad recently;y)

Question 19

Other than liver disease, when else might ALP be high?

Answer 19

Gall stone or cancer => blocked bile duct

Question 20

What can be measured to confirm any damage to heart muscle (e.g. due to a heart attack)?

Answer 20

Cardiac enzymes are present in the heart muscle and these enzymes leak into the blood in large amounts during a heart attack.

Question 21

Name four commonly measured cardiac enzymes

Answer 21

1. Troponins
2. Creatine kinase (CK)
3. Aspartate amino transferase (AST)
4. Lactate Dehydrogenase (LDH)