Introduction to Chemical Pathology

Question 1

List five common diagnostic tests carried out by the department of chemical pathology

Answer 1

1. Electrolytes (including, sodium and potassium).
2. Urea and creatinine. High levels suggest failure of renal excretion of these substances and hence renal failure. 
3. Calcium and phosphate 
4. Markers of liver function (liver enzymes). 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. Enzymes commonly measured include
   
   • alkaline phosphatase
   • aspartate amino-transferase (AST)
   • alanine amino-transferase (ALT)
   • gamma glutamyl transferase (GGT) 
     
     • One needs to measure AST and GGT in a patient with jaundice.
       
       • Aspartate transferase and gamma-glutamyl transferase
5. Hormone assays are done within a subdivision of the chem. path department (endocrinology). Hormones commonly measured include thyroxine, TSH and cortisol.

 

1. Glucose. This can be rapidly measured using a glucose sensitive stick which can be undertaken in the Ward/clinic/home. A more accurate method is carried out within the laboratory. Red cells will consume glucose, even after it is out of the patient, unless they are poisoned.
   
   • Red cells consume glucose (anaerobic glycolysis), so the longer this is left out, the lower the glucose may read.
   • Fluoride oxalate (poison) prevents the red cells from using glucose.
2. Cardiac Enzymes

• Are present in the heart muscle
• During a heart attack, heart muscle is damaged.
• These enzymes leak into the blood in large amounts
• Thus we can tell you if someone has really had a heart attack

Examples of Cardiac Enzymes

• Troponins
• Creatine Kinase (CK)
• Aspartate amino transferase (AST)
• Lactate dehydrogenase (LDH)
• Myoglobin

Summary of Tests

• FBC, ESR, CRP- FBC: Haemoglobin and white cell count
• LFTs- Liver leaks out enzymes all the time.
• U&Es- Urea and electrolytes
• Blood glucose
• Ix of viral illness
• Stool culture- bacteriology

Question 2

In response to a past paper question (Sourced from Oxford Handbook of clinical Medicine)

Answer 2

• Cardiac troponin levels are the most sensitive and are specific markers of myocardial necrosis. Serum levels rise within the first 3-12 hours, peak at 24-48 hours and fall to baseline over 5-14 days.
• Creatine kinase rise within 3-12 hours, peak at 24 hours and back to baseline at 48-72 hours.
• Myoglobin rises within 1-4 hours, very sensitive but not specific.

Question 3

Distinguish the tubes

Answer 3

• Purple tops contain EDTA which is a strong anti-coagulant and is therefore used for complete blood counts and blood films.
• Fluoride in the grey top stops glycolytic enzymes from functioning and therefore glucose is not used up during storage. The oxalate is the anticoagulant in this tube.
• Citrate is a reversible anticoagulant and is therefore used for coagulation assays.

Serum or plasma

• Serum is without the clotting factors
• Red cells can contaminate U&E because red cells contain a lot of potassium.
• Too much potassium causes asystole.

Question 4

Renal Failure

Answer 4

• Creatinine is a marker of glomerular filtration rate because very little is absorbed or secreted by the tubules. If it is normal, then the GFR is normal.
• Urea levels rise when a patient is dehydrated but GFR stays the same to the end.
• Serum creatinine is raised in renal failure as opposed to creatinine clearance, this is because the kidneys are unable to excrete creatinine and it builds up in the blood.

Question 5

Electrolytes:

• Na: 125 mM (NR 135 – 145) hyponatraemic
• K: 6.0 mM (NR 3.5 – 5.0) hyperkalaemic
• U: 12.4 mM (NR 2.5 – 6.7) very high urea
• Cr: 100 micromol/l (NR 70 – 120) normal creatinine

Answer 5

• Creatinine production rate is fixed and is a measure of muscle breakdown, because the creatinine is normal it means that the GFR is normal.
• Urea is a breakdown product of protein, obviously dehydration increases urea levels.
• The combination of hyponatraemia and hyperkalaemia could indicate hypoaldosteronism? Addison’s?- Adrenal Failure
• Potentially high potassium could be due to haemolysis of blood on withdrawal from the patient.
• Usually diarrhoea would decrease potassium.
• In renal failure the concentration of both urea and creatinine will rise.