Clinical biochemistry

Question 1

What is the definition of sensitivity and specificity

Answer 1

Sensitivity = ability to detect disease (positive in disease)
Specificity = ability to exclude disease (negative in health)

Question 2

What are examples of biological effects on a sample

Answer 2

Creatinine dependent on muscle mass
Urea dependent on protein intake and hydration status
Cortisol has diurnal variation
Testosterone/oestrogen/PSA dependent on age/gender

Question 3

What are examples of pre-analytical effects on a sample

Answer 3

Sample identification
Timing of sample collection (e.g. for cortisol)
Sample tube
Infusion arm sampling

Question 4

What are examples of analytical effects on a sample

Answer 4

Haemolysis
Icteric samples
Lipaemic samples
Bubbles

Question 5

Compensation is the attempt to turn acid-base status back to normal by what mechanisms

Answer 5

Buffering-
Bicarb in serum, phosphate in urine
Skeleton
Intracellular accumulation/loss of H ions

Compensation-
Diametric opposite of the original abnormality
Never overcompensates
Delayed and limited

Treatment-
By reversal of the precipitating situation

Question 6

How long does respiratory compensation for a primary metabolic defect take to occur

Answer 6

Rapidly

Question 7

How long does metabolic compensation for a primary respiratory defect take to occur

Answer 7

Slower- 36-72hrs

Question 8

What does a hyperkalaemia cause

Answer 8

Acidosis

Question 9

What does a hypokalaemia cause

Answer 9

Alkalosis

Question 10

List causes, compensation, correction and features of respiratory acidosis

Answer 10

CAUSES-
Anything that stops gas exchange
Airway obstruction- bronchospasm, COPD, aspiration, strangulation
Respiratory centre depression- anaesthetics, sedative, cerebral trauma, tumours
Neuromuscular disease- Guillain-Barre syndrome, motor neurone disease
Pulmonary disease- pulmonary fibrosis, RDS, pneumonia
Extrapulmonary disease- flail chest

COMPENSATION-
Increased renal acid excretion

CORRECTION-
Requires return of normal gas exchange

FEATURES-
Acute= low pH, high H, normal HCO3, high pCO2
Chronic= low pH, high H, high HCO3, high pCO2

Question 11

List causes, compensation, correction and features of respiratory alkalosis

Answer 11

CAUSES- 
Hypoxia- high altitude, severe anaemia
Pulmonary disease- oedema, embolism
Mechanical overventilation
Increased respiratory drive- salicylates, trauma, infections, tumours, hepatic failure, gram negative septicaemia, primary hyperventilation syndrome

COMPENSATION-
Increased renal bicarbonate excretion

CORRECTION-
Of cause

FEATURES-
Acute= high pH, low H, normal HCO3, low pCO2
Chronic= high pH, low H, low HCO3, low pCO2

Question 12

List causes, compensation, correction and features of metabolic acidosis

Answer 12

CAUSES-
Increased acid formation- ketoacidosis, lactic acidosis, poisoning
Acid ingestion- poisoning
Reduced acid excretion- renal tubular acidosis, renal failure, carbonic dehydratase inhibitors
Loss of bicarbonate- diarrhoea, pancreatic/intestinal/biliary fistula

COMPENSATION-
Hyperventilation

CORRECTION-
Increased renal acid excretion

FEATURES-
Low pH, high H, low HCO3, low pCO2

Question 13

List causes, compensation, correction and features of metabolic alkalosis

Answer 13

CAUSES-
Increased loss of acid- vomiting
Increased renal H excretion- diuretics, mineralocorticoid excess

COMPENSATION–
Hypoventilation with CO2 retention

CORRECTION-
Increased renal bicarbonate excretion, reduced renal protein loss

FEATURES-
High pH, low H, high HCO3, normal pCO2