Chemical Pathology

Question 1

What components in blood might cause a false hyponatraemia?

Answer 1

Mannitol, ethanol, lipids

Question 2

Recall 3 causes of hypervolaemic hyponatraemia

Answer 2

Heart/ renal/ liver failure

Question 3

Recall 3 causes of euvolaemic hyponatraemia

Answer 3

Hypothyroidism, glucocorticoid insufficiency, SIADH

Question 4

Recall 3 tests to do in euvolaemic hyponatraemia to determine the cause

Answer 4

TFTs, short synacthen, paired serum + urine osmolality

Question 5

How should you manage hypervolaemic hyponatraemia?

Answer 5

Fluid restrict, treat the cause

Question 6

Recall some causes of hypovolaemia

Answer 6

D&V, diuretics, salt losing nephropathy?

Question 7

How should hypovolaemic hyponatraemia be managed?

Answer 7

Fluid restrict with 5% dextrose

Question 8

What must always be ruled out before diagnosing SIADH?

Answer 8

Thyroid/ cortisol causes - so check both - SIADH is a diagnosis of exclusion

Question 9

How can the cause of hypovolaemic hyponatraemia be determined?

Answer 9

Urinary sodium
If >20 = renal causes
If <20 = D&V

Question 10

How can urinary sodium narrow down your differentials in hypervolaemic hyponatraemia?

Answer 10

If sodium >20 = renal cause (kidneys can’t hold onto sodium even though it’s low) - so CKD/ AKI

If sodium <20 (low) = heart/ liver failure

Question 11

What are the 4 criteria needed to diagnose SIADH?

Answer 11

1. True hyponatraemia (low serum osmolarity AND osmolality)
2. Clinically euvolaemic
3. Diagnosis of exclusion (normal TFTs and 9am cortisol)
4. High urine osmolality

Question 12

Which drug can be used in SIADH if treating the cause is insuffucuent and what is their MOA?

Answer 12

Demeclocycline

Decreases sensitivity of AQP2 channels

Question 13

What is the main risk of rapid correction of hyponatraemia?

Answer 13

Central pontine myelinolysis

Question 14

Recall 4 causes of hypernatraemia

Answer 14

D&V (if you’re mostly losing water)
Diabetes insipidus
Conn’s (aldosterone)
Iatrogenic

Question 15

What investigation is most useful for confirming a diagnosis of Addisson’s?

Answer 15

Aldosterone:renin ratio

Question 16

Recall 3 possible causes of nephrogenic diabetes insipidus?

Answer 16

Lithium
Hypercalcaemia
Hypokalaemia

Question 17

What is the best investigation to do when trying to determine the cause of diabetes insipidus?

Answer 17

Fluid deprivation test

Question 18

What are the steps of a fluid deprivation test?

Answer 18

1. Fluid restrict (no drinking for 8 hours) and record urine osmolality
2. Give desmopressin and measure urine osmolality again

Question 19

What would be the result of a fluid deprivation test in polydipsia, cranial DI and nephrogenic DI?

Answer 19

PP: urine immediately concentrates

CDI: urine only concentrates after desmopressin given

NDI: urine never concentrates

Question 20

Bartter syndrome has the same effect of WHICH type of diuretic?

Answer 20

Thiazide

Question 21

Recall the treatment for mild and severe hypokalaemia

Answer 21

Mild: SandoK (2 tablets TDS for 3 days)
Severe: IV KCl

Also treat cause and CORRECT Mg

Question 22

Recall 3 symptoms of hypokalaemia

Answer 22

• Muscle weakness
• cardiac arrhythmia
• Polyruia/polydipsia (nb: one of the causes of nephrogenic DI is hypokalaemia)

Question 23

At what rate can KCl be given IV?

Answer 23

<10mM per hour

Question 24

What is the effect of spironolactone on potassium?

Answer 24

Inhibits aldosterone –> less K+ loss –> hyperkalaemia

Question 25

Recall systematically some causes of hyperkalaemia

Answer 25

1. Spurious sample
2. Extracellular shift (rhabdomyolysis, acidosis)
3. Decreased excretion (renal: AKI, CKD, drugs: spironolactone, ACE inhibitors; Addisson’s - not getting effect of aldosterone –> losing sodium –> high potassium)

Question 26

Recall some investigations to do in hyperkalaemia

Answer 26

1. VBG (acidosis?)
2. CK
3. Creatinine and urea