Case # 16

Ion Selective Electrodes

• • During a routine analysis of electrolytes a technologist

notes that one set of results is unusually low:• Sodium 118 mEq/L• Potassium 3.8 mEq/L• Upon repeat analysis similar results are·obtained. • Both·cations are measured by dilutional ion selective

electrodes.• A check in the computer shows that a previous result from a

sample taken after an overnight fast had normal sodium and potassium concentrations.

What are the principles of the most commonly used techniques for measuring Na and K?

• Direct Ion Selective Electrode (ISE) (No dilution of specimen)• Indirect ISE

Specimen is Diluted• Old method Flame Photometry

Specimen is diluted and is a “Mass” measurement

• Ion selective electrodes (ISE) measure the activity of Na and K, which is related to concentration.

• The activity measurement is based on the voltage difference established on the ISE in the presence of the specific ion

• The ISEs perform the analysis directly on the sample (non-dilutional ISE) or following a dilution of the sample (dilutional ISE)

What is the difference between “activity” and mass concentration measurements?

• In activity measurements, only the amount of material that is physiologically and chemically available - i.e., active - is measured

• If the sample is diluted before being brought into contact with the ISE, that dilution must also be accurately performed

What is the most likely cause of such low and most probably factitious sodium results?

• Lipemia or very elevated Protein• The lipid expands the volume of the

plasma although the concentration of sodium and potassium in the plasma water remains constant, as the cations do not readily dissolve in the lipid frac tion of blood.

How can one quickly determine if one of the causes of factitious hyponatremeia is present?

• Look at the specimen• A lipemic sample will appear “milky”• PSEUDOHYPONATREMIA• Refractometer will quickly give a total protein

estimation

Why is the potassium result not as proportionally low as the result for sodium?

• The Width of the range/Middle of the range x 100 =.• • Sodium Reference Range NA (135 – 145)• • 10/140 x 100 = 7%• • Potassium Reference Range K (3.5-5.0)• • 1.5/4.25 x 100 = 35%• • The reference range for K (35%) is wider than the reference range for

sodium (7%)

Effect of Dilution10 9Na Na Na NaNa Na Na NaNa Na Na NaNa Na Na NaNa Na Na Lipid

Reference range for K (35%) is wider than the reference reference range for Na (7%)

Na Normal Range 135 – 145 140 x 0.1 = 14Na 140 – 14 = 126 Hyponatremia

K Normal Range 3.5 – 5.0 4.25 x 0.1 = 0.425 4.25 – 0.425 = 3.825 Still in normal range

Lipemic Specimenhttp://liposol.com/