Chemical pathology: blood gas and electrolyte measurements

Question 1

What are the four major electrolytes in blood?

Answer 1

Sodium ions, potassium ions, chloride ions and bicarbonate ions

Question 2

What is the reference range of sodium ion?

Answer 2

135 - 145 mmol/L

Question 3

What is the reference range of potassium ion?

Answer 3

3.5 - 5.2 mmol/L

Question 4

What is the reference range of chloride ion?

Answer 4

95 - 110 mmol/L

Question 5

What is the reference range of bicarbonate ions?

Answer 5

22 -32 mmol/L

Question 6

What is the best way to collect blood for a reliable potassium result?

Answer 6

Collect blood with heparin (substance that stabilise red cell membrane), maintain it at room temperature and separate plasma/serum within 1 hour by high speed ultracentrifugation without cooling.

Question 7

Why is potassium ion measurement affected by degree of hemolysis?

Answer 7

Potassium ion measurement refers to the amount of potassium ions present in the extracellular fluid (ECF), therefore in the presence of hemolysis (red cell breakdown), a falsely raised potassium ion can be measured as red cell membrane releases potassium ions to ECF when breaking down.

Question 8

What is the methodology used to measure sodium ions?

Answer 8

1. Atomic absorption photospectrophometry AAS
2. Electrochemically using sodium ion - ISE
3. Spectrophotometric assay

Question 9

What is the methodology used to measure potassium ions?

Answer 9

1. Potassium ion - ISE

2. Spectrophotometric assay

Question 10

Sodium and potassium ions can be measure with spectrophotometric methods. Describe 2 ways in which it can be done.

Answer 10

1. Enzyme activation

2. Detecting spectral shift produced when sodium and potassium ions bind to a macrocyclic chromophore.

Question 11

What is the methodology used to measure chloride ion?

Answer 11

1. Mercurimetric titration
2. Spectrophotometric assay
3. Colourimetric-amperometric titration
4. Chloride ion - ISE

Question 12

How does ISE (ion selective electrode) work?

Answer 12

ISEs contain potentiometric devices that determine the change in electromotive force in a circuit between a measurement electrode (ISE) and a reference electrode, as the selected ion interacts with the membrane of the ISE. The potential of the calibrators provide the information to generate a graph of change in electromotive force against the change in log of concentration response, are stored and used as a comparison for calculating unknown concentration when E of the unknown is measured.

Question 13

What is the difference direct and indirect ISE?

Answer 13

During indirect ISE, the sample is into the measurement chamber after mixing with a large volume of diluent. During direct ISE, the sample is introduced to the electrodes without a dilution. In indirect ISE the dilution is calculated according to the conventional 93.3% water present in plasma. In direct ISE, the measurements need to be calculated to the total volume plasma using flame mode.

Question 14

What are the common errors involved with ISEs?

Answer 14

Errors related to ISEs are due to a lack of selectivity, repeated protein coating of the ion-sensitive membranes or contamination of the membrane or salt bridge ions.

Question 15

Provide one of the limitations of indirect ISE.

Answer 15

Errors related to ISEs are due to a lack of selectivity, repeated protein coating of the ion-sensitive membranes or contamination of the membrane or salt bridge ions.

Question 16

Provide one of the limitations of direct ISE.

Answer 16

One of the limitations of direct ISE is that it is affected by the ionic strength of the sample, and changes to the surface of the electrodes from plasma components.