Chemical pathology

Question 1

List routine tests in clinical medicine. (x8) [Need to know these, but don’t worry about listing them all; they will all be covered individually.]

Answer 1

• Full blood count. • ESR – erythrocyte sedimentation rate. • CRP. • LFTs – liver function tests. • U&Es – urea and electrolytes. • Blood glucose. • Illness of viral illness. • Stool culture – bacteriology.

Question 2

What is an ESR?

Answer 2

Erythrocyte sedimentation rate test that measures how quickly erythrocytes settle at the bottom of a test tube. Normally, RBCs settle relatively slowly; a faster-than-normal rate may indicate INFLAMMATION in the body.

Question 3

What is CRP in laboratory diagnostics?

Answer 3

C-reactive protein: Made by the liver when you have an infection.

Question 4

What are liver function tests?

Answer 4

Liver contains lots of enzymes. In liver function tests, we test the levels of three liver enzyme levels. In a healthy liver, there is tiny amounts of leakage of enzymes into the blood. If the liver is damaged, more of these enzymes leak out into the blood.

Question 5

How can liver function tests tell us the kind of liver disease a patient has?

Answer 5

The pattern of the leak of enzymes in the patient.

Question 6

What is measured in a liver function test? (x4)

Answer 6

• Albumin – synthesised in the liver. In liver disease, less Albumin is produced and so less is found in the blood. In the others listed below, levels would be high in the blood in a diseased liver patient. • Bilirubin – produced in the liver when Hb is broken down. It is normally excreted into the gall bladder as BILE, and colours faeces. If something is stopping this, it leaks into the blood –> jaundice. • Alkaline Phosphatase – affects flow of bile. • ALT (alanine amino-transferase) – produced and leaks out in inflamed liver e.g. Hepatitis.

Question 7

When a patient is jaundiced, what should be measured alongside a liver function test? (x2)

Answer 7

AST (aminotransferase) and GGT (gamma-glutamyl transferase) – would be high in the blood in a diseased liver.

Question 8

What are ‘U&E’s?

Answer 8

Measure for urea and electrolytes in the blood. It is a measure of renal function.

Question 9

Blood collection tubes have different colour tops: What do these colour tops indicate? (x6) !!!

Answer 9

• Different colour indicates that tubes have DIFFERENT ANTICOAGULANTS. • RED: no anti-coagulants – so all clotting factors in the blood are used up in the sample, leaving serum. • YELLOW: have gel to speed up clot – speeds up clotting so serum separates faster. • PURPLE: potassium EDTA. • GREY: have fluoride oxalate (poison) – prevent RBCs from undergoing glycolysis. • BLUE: contains citrate (anticoagulant), so preserves clotting factors for measurement. • GREEN: contains lithium.

Question 10

What is the importance of using grey top tubes?

Answer 10

Fluoride oxalate prevents glycolysis in red blood cells by poisoning them. If glycolysis was not prevented, red cells would consume glucose in the plasma (anaerobic glycolysis), so the longer the sample is left out, the lower the blood glucose may read. This could mean that hyperglycaemia is missed etc.

Question 11

What is the difference between serum and plasma?

Answer 11

Serum is the liquid component of blood left after blood has CLOTTED; plasma is the liquid component that remains when clotting is prevented with the addition of an anticoagulant – separated instead by centrifuge. Therefore, plasma contains CLOTTING FACTORS, and serum does not.

Question 12

Which blood collection tubes are used for U&E tests?

Answer 12

Placed in yellow/red top and SERUM tested.

Question 13

Which blood collection tubes are used for glucose tests?

Answer 13

Placed in grey top tube and PLASMA tested.

Question 14

Which blood collection tubes are used for HbA1c tests?

Answer 14

PLASMA is collected in purple top tube.

Question 15

Which blood collection tubes are used for TFT (thyroid function) tests?

Answer 15

SERUM is collected in yellow/red top tubes.

Question 16

Which blood collection tubes are used for liver function tests?

Answer 16

Yellow/red top tubes.

Question 17

How is serum isolated so that it can be tested from a blood sample?

Answer 17

Blood sample is placed in a red/yellow top tube and blood clots, leaving serum. The sample is centrifuged, so Hb (which is heavy) sinks to the bottom of the tube, while yellow-looking serum floats and can be extracted by drawing up in a needle. The opaque liquid in the middle is the gel that speeds up coagulation in the yellow tube and prevents movement of RBCs into the serum-separated layer.

Question 18

How is plasma isolated so that it can be tested from a blood sample?

Answer 18

Anticoagulant such as EDTA or heparin is used to prevent blood from clotting, and it is centrifuged. The photo indicates what these separates. Plasma can subsequently be extracted by drawing up the top layer using a needle/syringe.

Question 19

What are blood sample collection tubes called?

Answer 19

Vacutainers.

Question 20

Who are chemical pathologists?

Answer 20

• In charge of chemical pathology laboratories. • Research into better methods of getting results.

Question 21

When do you need to contact a chemical pathologist? (x3)

Answer 21

• When you want the sample to be rapidly centrifuged out of hours i.e. emergency. Otherwise, it would just be left until the morning and it would take a long time to know results. • When you want to measure labile (easily broken down) hormones such as insulin (insulin is destroyed from clotting). • When you urgently need CSF glucose and protein (cerebrospinal fluid) to be measured. If glucose is low and protein is high, it is indicative of bacteria presence in the CSF (because bacteria produce protein and use up glucose).

Question 22

What is measured in an electrolyte test?

Answer 22

Sodium, potassium, calcium and chloride.

Question 23

What is creatinine a measure of?

Answer 23

Marker of GFR; if normal, then GFR is normal – very little is absorbed or secreted by tubules.

Question 24

What may be the causes of high potassium levels in the blood? (x3)

Answer 24

• Adrenal failure – failure to produce aldosterone which promotes sodium reabsorption and potassium excretion in the kidneys. • Renal failure – loss of excretion of potassium in the kidneys, so levels climb in the blood. • Red cell haemolysis – if venepuncture is poor and you damage the patient i.e. your venepuncture is not efficient, you risk destroying red cells. RBCs contain a lot of potassium, so haemolysis can increase potassium in the sample and wrongly indicate high system levels too.

Question 25

How is renal function assessed?

Answer 25

Measuring sodium, potassium, creatinine and urea.

Question 26

What is indicative in the blood of renal failure? How does this link to the disease pathophysiology?

Answer 26

• Low sodium and low BP from loss of homeostatic function of the kidneys. When homeostatic function of kidneys is lost, kidney cannot reabsorb the molecules it filters. Hence, sodium and water are both lost from the blood. • High potassium and urea from loss of excretory function in the kidneys. • High creatinine from reduced GFR in the kidneys.

Question 27

How do urea and creatinine levels compare in renal failure and severe dehydration?

Answer 27

• RENAL FAILURE: creatine and urea levels are both HIGH because kidneys are less/not functional, so GFR is reduced and excretion capacity also limited. • DEHYDRATION: creatine levels are NORMAL because kidneys are working fine, but urea is HIGH because more is moved into the medulla interstitium (and therefore reabsorbed) in an effort to increase the osmotic gradient and increase reabsorption of water. Creatine levels do however, rise when dehydration is very severe.

Question 28

How are cardiac enzymes used in laboratory diagnostics?

Answer 28

Cardiac enzymes are found in the heart muscle. During a heart attack, heart muscle is damaged and these enzymes LEAK into the blood in large amounts. Thus, we can tell you if someone is having a bad heart attack.

Question 29

Why may laboratory diagnostics be important in diagnosing a heart attack?

Answer 29

When a patient does not feel pain so cannot feel that they are having a heart attack e.g. diabetic sensory neuropathy.

Question 30

What cardiac enzymes are measured in laboratory diagnostics? (x4)

Answer 30

• Troponins. • Creatine kinase (CK) • Aspartate amino transferase (AST) – note that these are released by the liver also! • Lactate dehydrogenase (LDH).

Question 31

What does a typical chemical pathology request form look like? (x3 components)

Answer 31

□ Patient details e.g. name, hospital number and DOB.

□ Clinical details e.g. name, ward and bleep number.

□ Test details – checklist of all required tests.