1. Ion Selective Electrodes
Case # 16
Ion Selective Electrodes

2. During a routine analysis of electrolytes a technologist notes that one set of results is unusually low:
Sodium mEq/L
Potassium 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.

3. Direct Ion Selective Electrode (ISE) (No dilution of specimen)
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

4. 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)

5. 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

6. 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 frac­tion of blood.

7. 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

8. 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 ( )
1.5/4.25 x 100 = 35%
The reference range for K (35%) is wider than the reference range for sodium (7%)

9. Effect of Dilution 10 9
Na 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 = 14
Na 140 – 14 = Hyponatremia
K Normal Range – 5.0
4.25 x 0.1 =
4.25 – = Still in normal range

10. Lipemic Specimen http://liposol.com/