Biochemistry Flashcards
Which 2 terms are used to describe how good a biochemical test is?
- Accuracy: How close the result is to the true value
- Precise: how consistent/predictable the results are
Reference interval
If we measure something in a population or sample of people, there will be a central tendency and a scatter around that central tendency
False negative
a person with the disease whose result is in the ‘healthy’ territory and which is therefore falsely reassuring
False positive
A healthy person whose result is in the ‘disease’ territory and which therefore acts as a false alarm
Sensitivity
Number of those with the disease who were detected
(True positives/True positives + False negatives)
Specificity
Of those who were detected, how many actually had the disease?
(True negatives/True negatives + false positives)
(Ratio of the no people detected as not having it comared to the number of people who actually didnt have it)
What is being regulated in acid-base balance?
Hydrogen ion concentration ([H+], pH)
Why is bicarbonate so important in acid-base balance?
It is the only buffer for H+ that doesnt reach equilibrium, the only limit is initial concentration of HCO3- that is available to buffer
With all acid-base imbalances, what are the 2 stages of insult?
- Primary issue from the imbalance
- Secondary issue of compensation
Acidaemia
increased [H+]
Alkalaemia
decreased [H+]
Acidosis
Process tending to cause increased [H+]
Ratio of CO2:bicarbonate rises. Can occur either because the ratio of CO2 increases, or if the bicarbonate decreases due to mopping up the acid load.
Alkalosis
Process tending to cause decreased [H+]
Ratio of CO2:bicarbonate falls. Either the CO2 decreases due to hyperventilation to compensate for an area of the lungs not being available and so CO2 is blown off, or if the bicarbonate. concentration increases.
What is the respiratory component?
pCO2
pCO2 increases = Respiratory acidosis
pCO2 decreases = Respiratroy alkalosis
What is the metabolic component?
HCO3-
HCO3- decreases = Metabolic acidosis
HCO3- increases = Metabolic alkalosis
What is:
1) Respiratory acidosis
2) Respiratory alkalosis
3) Metabolic acidosis
4) Metabolic alkalosis
What are the compensations for each of the primary acid-base disorders?
Respiratory acidosis and is method of compensation
Primary problem is too much CO2 (because lungs cant shift the CO2 due to disease)
Compensate by excreting more H+ in the kidneys
Metabolic acidosis and its method of compensation
Primary problem is too much H+
Compensate by blowing off CO2
Causes of respiratory acidosis (too much CO2)
- Choking
- Bromchopneumonia
- COPD
Causes of respiratory alkalosis (not enough CO2)
Hyperventilation (blowing off excess CO2) due to:
- Hysterical over-breathing (panicking)
- Mechanical over-ventilation
- Raised intra-cranial pressure - stimulates the respiratory centre and causes over-breathing
Causes of metabolic acidosis (decreased HCO3-)
- Impaired H+ excretion
- Increased H+ production or ingestion (e.g. anti-freeze overdose)
- Loss of bicarbonate (dudodenal fluid)
Causes of metabolic acidosis (too much HCO3-)
- Loss of H+ in vomit (vomit is obviously acidic)
- Alkali ingestion
- Potassium deficiency
True or False: You can overcompensate in acid-base balance
False, you can never over-compensate ie. you cant become alkolotic by compensating for acidosis
How should you approach an acid-base balance problem?
- 1) Check the history for clues
- 2) Check H+ - are they acidotic or alkalotic?
-
3) Then check for primary cause:
- If acidotic:
- Is their CO2 raised? = Respiratory acidosis
- Or is their HCO3 lowered? = Metabolic acidosis
- If alkalotic:
- Is their CO2 lowered? = Respiratroy alkalosis
- Is their HCO3- rasied? = Metabolic acidosis
- If acidotic:
-
4) Then check for signs of compensation:
- If respiratory, check HCO3 for changes
- Ie. Respiratory acidosis > HCO3 increased
- Respiratroy alklaosis > HCO3 decreased
- If metabolic, check PCO2 for changes
- ie. Metabolic acidosis > PCO2 decreased
- Metabolic alklaosis > PCO2 increased
- If respiratory, check HCO3 for changes
-
5) Finally, check for degree of compensation
- If there are no changes in opposite component (ie. HCO3 normal in respiratory acidosis) then uncompensated
- If slight changes in opposite but H+ still isnt normal = partially compensated
- If the opposite factor is extreme high/low, and everything else is normal = Fully compensated
- 31yo woman during acute asthmatic attack
- [H+] = 24 nmol/L (36-44 is normal)
- pCO2 = 2.5kPa (4.7-61)
- [HCO3-] = 22mmol’/ (22-30)
What is the diagnosis?
- Her H+ is low, so is alkalotic
- PCO2 is low, so is respiratory alkalosis
- Then check for compensation: HCO3- is normal so she is not in metabolic acidosis, so is uncompensated
Therefore: Uncompensated respiratory alkalosis
- 16yo with type 1 diabetes
- Admitted with collapse, breath smells funny and noisy breathing
- Glucose = 33.2 mmol/L
- [H+] = 73nmol/L (36-44)
- HCo3-] = 5mmol/L (22-30)
- pCO2 = 1.0 kPa (4.7-6.1)
What is the diagnosis?
- Based on history would expect him to have metabolic acidosis due to diabetic ketoacidosis
- H+ is high, so is acidotic
- HCO3 is low and PCO2 isnt high, so is metabolic acidosis
- PCO2 is low, so is compensating with respiratory alkalosis
- But H+ isnt normal, so only partially
Therefore, diagnosis is partially compensated metabolic acidosis
- 71yo man with stable COPD
- [H+] = 44nmol/L (36-44)
- pCO2 = 9.5 (4.7-6.1)
- [HCO3-] = 39 mmol/L (22-30)
What is the diagnosis?
- Based on the history, would expect him to be respiratory acidotic as cant blow off CO2
- H+ is normal - but slightly high side of normal so was compensated but acidotic?
- PCO2 is high and HCO3 isnt low so is respiratory acidosis
- Compensation: HCO3 is very high so compensating with metabolic alkalosis
- H+ is normal so fully compensated
Therefore, diagnosis is fully compensated respiratory acidosis
- 23yo man with dyspensia and excess alchol who’s been vomiting for 24hrs
- [H+] = 28nmol/L (36-44)
- [HCO3-] = 48 mmol/L (22-30)
- pCO2 = 9.5 (4.7-6.1)
What is the diagnosis?
- Based on the history, would expect him to be metabolically alkalotic as been vomiting H+
- H+ is low so is alkalotic
- HCO3 is high and PCO2 isnt low so is metabolic alkalosis
- Compensation: PCO2 is high so is compensating with respiratory acidosis
- H+ isnt normal however so is only partial
Therefore, diagnosis is partially compensated metabolic alkalosis
- 50yo man with 2 week history of vomiting and diarrhoea
- Dry, deep noisy breathing
- [H+] = 64nmol/L (36-44)
- [HCO3-] = 8 mmol/L (22-30)
- pCO2 = 2.8 (4.7-6.1)
What is the diagnosis?
- Sounds like acidotic breathing so potentially metabolic acidosis?
- H+ is high so acidotic
- HCO3 is low and CO2 isnt high so is metabolic acidosis
- Compensation: pCO2 is low so is compensating with respiratory alkalosis
- H+ isnt normal so is only partial
Therefore, diagnosis is partially compensated metabolic acidosis
What is the body’s optimim pH?
Between 7.35 and 7.45
What must the PaO2 (PO2) be interpreted against?
Inspired oxygen concentration
What is base excess (BE)?
A measure of how much base as to be removed or added to the system to correct the pH in standard conditions (it assumes normal CO2)
