Chemical Pathology 12 - Meeran's Diabetes CPC

Question 1

What are the biochemical definitions of diabetes?

Answer 1

Fasting PLASMA glucose >7.0mM (nb. this value does not apply to fingerprick whole blood test which just needs to be > 6.1)

HbA1c > 6.5% (equivalent >48mmol/mol)

2 hour plasma glucose in 75g Oral Glucose Tolerance Test of > 11.1mM

Random glucose > 11.1mM

Question 2

What HbA1c values count as ‘impaired glucose tolerance’?

What is impaired glucose tolerance in the OGTT?

What is impaired fasting glucose?

Answer 2

42-48 mmol/mol

7. 8-11.1 mmol/L
8. 1-7.0 mmol/L

Question 3

Recall 3 differentials for metabolic alkalosis

Answer 3

H+ loss via vomiting (see history)
Hypokalaemia
Bicarb ingestion (rennies - rare nowadays since the invention of omeprazole)

Question 4

How can anion gap assist in diagnosis of DKA?

Answer 4

Ketones are anions

Therefore, in DKA anion gap will be large

Question 5

How can pituitary-dependent Cushing’s and ectopic ACTH be distinguished?

Answer 5

Pituitary petrosal sinus sampling

Question 6

If a patient has a high ACTH and very severe hypokalaemia, what is the most likely cause of the high ACTH?

Answer 6

Ectopic ACTH

Question 7

What test is best to diagnose the cause of ectopic ACTH?

Answer 7

CXR

Question 8

How can acute and chronic renal failure be distinguished?

Answer 8

Renal biopsy

Question 9

How should acute tubular necrosis be managed?

Answer 9

Dialyse for 3 weeks and they willl recover

Question 10

How should diabetic glomerular kidney disease be managed?

Answer 10

This is a lifelong condition that will require lifelong dialysis

Question 11

What is the difference in expected pCO2 in uncompensated metabolic and respiratory acidosis?

Answer 11

Metabolic: low pCO2 (equilibrium pushed right to produce more CO2 but this is breathed off nicely)

Respiratory: high pCO2 (not ventilating properly to get rid of CO2)

Question 12

What is the difference in expected pCO2 in uncompensated metabolic and respiratory alkalosis?

Answer 12

Metabolic: high pCO2 (reduced H+ means resp rate decreases to produce more CO2 to replace H+)

Respiratory: low pCO2 (hyperventilation –> blowing off all CO2)

Question 13

How can hypoglycaemia lead to a respiratory alkalosis?

Answer 13

Can cause significant anxiety –> hyperventilation

Question 14

Why can very high serum omolality cause unconsciousness?

Answer 14

Brain gets VERY dehydrated

Patients with undiagnosed type 2 diabetes may present with a hyperosmolar hyperglycaemic state

Question 15

What metabolic imbalance can be caused by metformin?

Answer 15

Metformin is thought to inhibit the conversion of lactate into glucose (Cori cycle) -> in excess/impaired renal excretion, this can lead to lactic acidosis

(These lactate molecules are anions so will cause high anion gap, but urine will be negative for ketones)

Question 16

How is diabetes diagnosed?

Answer 16

Symptoms + 1 diabetes test result

No symptoms + 2 diabetes test results

Question 17

How is osmolality calculated?

NEED TO KNOW THIS FOR THE EXAM

Answer 17

Charged + uncharged molecules = cations + anions + urea + glucose

Cations = anions so this is reduced to 2 x (Na + K) + urea + glucose

• We don’t know the measurements of all of the anions present so we assume that the total is the same as the cations*
• The other cations aside from sodium and potassium are very minor players so they are disregarded*

Question 18

How do you calculate anion gap?

Answer 18

Cations (Na + K) = Anions (Cl + Bicarb + others)

The “others” are known as the anion gap

Anion gap = Na + K - Cl - Bicarb

High anion gap suggest extra “other” anions e.g. KETONES, LACTATE, methanol, ethanol

Question 19

Why does hyperventilation cause tetany?

Answer 19

Hyperventilation causes alkalosis

As pH rises, albumin becomes “stickier” and more calcium binds to albumin

Low free ionised calcium -> tetany