1. Potassium A Discussion of how we measure Potassium concentration in the blood and the measures we take to quality control this technique. Including an overview of the role of Potassium and the effects of an imbalance in the blood. Laura Duffy DT204/2

2. Potassium in the Human Body An alkali metal element present as a cation. Present in all cells. Involved in establishing a cell's membrane potential, and creates action potential in nerve signal transmission. Transported actively between intracellular and extracellular fluid to establish osmotic gradient and achieve fluid balance.

3. Figure 1. Solid elemental Potassium. Image credit: Parsons and Dixon 1

4. Specimen Acquisition Extracellular Potassium concentration is relatively much lower than that inside the cells. Standard specimen is blood (whole, serum or plasma) and Potassium is usually included in a U&E investigation. Most often used yellow serum separator tube – gel barrier, ready for centrifugation. 2

5. Figure 2: Serum separator tube for standard U&E testing Photo credit: www.medicalexpo.com

6. Ion-Selective Electrodes Ion selective electrodes used for most potassium analyses. Machine contains potassium-specific electrode and reference electrode. Electrical potential created between the two when submerged in the serum. Output potential is then measured and is proportional to the activity of potassium. Activity translated to concentration using a calibration curve.

7. Difficulties in the Lab One of the most sensitive tests to artefactual change! Reference range: 3.5 - 5.0 mmol/L 2 * Potassium present in the haematocrit can be released during sample collection and/or transit and increase the potassium concentration of the plasma: A false result for hyperkalaemia

8. Quality Control Samples need to be taken correctly, transported without delay at a stable suitable temperature and processed as soon as possible in the lab, ideally without chilling. Samples should be spectrophotometrically inspected for evidence of haemolysis, which is integrated in most automated devices. Known standard stock concentrations are used as controls. These can be prepared on site or bought pre-made.

9. Quality Control All patient samples must be diluted in suitable ionic strength adjustment buffer, e.g. 10M Tetraethylammonium chloride. 3 Interfering ions: Rubidium, Caesium: Not present in serum Sodium, Lithium: To a lesser extent due to low selectivity coefficient. This increases as Potassium concentration decreases. 3

10. Hypokalaemia Causes: Malabsorption, increased peristalsis, resectioned bowel etc. Excess aldosterone, e.g. Conn's syndrome Metabolic alkalosis Consequences: Weakness, fatigue, general lethargy. Decreased muscle activity, including symptoms such as constipation and hypotension. Untreated prolonged deficiency can cause cardiac arrhythmias, paralysis and MI. 4

11. Hyperkalaemia Causes: Increased cellular release. Insufficient aldosterone released. Chronic renal disease or SLE. Insufficient insulin in unmanaged diabetes mellitus. Metabolic acidosis. Consequences: Weakness and fatigue Bradycardia and heart arrhythmia, can lead to ventricular fibrilation. Prolonged extremely high levels may cause cardiac arrest.

12. Reference List 1. Parsons, P and Dixon, G., The Periodic Table: A Field Guide to the Elements, London: Quercus, 2013 2. Cameron, D., Biochemical Investigations and Quality control, In: Ahmed, N., Clinical Biochemistry, New York: Oxford, 2011 3.Thermo Fisher Scientific Inc. User Guide: Potassium Ion Selective Electrode, 2008. Available from www.thermoscientific.com www.thermoscientific.com 4.Martini et al., Fundamentals of Anatomy and Physiology 9 th, London: Pearson, 2014