4.13: Electrolytes

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A medical technologist obtained the following results on a routine renal profile:

Sodium: 139 mmol/L
CO₂: 20 mmol/L
Potassium: 4.1 mmol/L
BUN: 190 mg/L
Chloride: 118 mmol/L
Creatinine: 9 mg/L
Glucose: 2500 mg/L

The sodium, potassium, and CO₂ are measured by ion selective electrodes. The chloride is measured by the ferric thiocyanate method. Upon reviewing the results, the medical technologist performs a quick written calculation and decides that the specimen should be reanalyzed, believing that the results are unusual.

QUESTIONS

What simple calculation can be used to check the validity of the electrolyte results? What can the technologist conclude from these results?
The technologist repeats the analysis. Essentially the same results are found. The technologist then examines the sample, and upon looking at it, repeats the chloride analysis by an alternative method. What do you think was the positive interference in this sample?

Questions to Consider

Which of the above results seem unusual?
What other result(s) can be calculated from these data?
The technologist calculated the anion gap for these results. Calculate the anion gap.
What are some possible causes of the calculated anion gap?
Can the anion gap be used to evaluate the validity of electrolyte analysis?
What are possible positive interferences in the ferric thiocyanate method for chloride?
What alternative method for chloride analysis reduces the effect of these positive interferent was probably?

Answer

To check the validity of the chloride result, the technologist calculated the anion gap. As discussed above in #5, the most likely cause of a very low anion gap is analytical error. The very low anion gap for these results suggested to the technologist that the chloride result was probably not accurate.
Upon examining the sample, the technologist probably saw a highly lipemic specimen. Knowing that lipemia can lead to falsely elevated chloride results, the technologist repeated the chloride analysis by a method less affected by lipemia, i.e. the chioridometer. If the specimen was clear and the result by the chioridometer was still elevated, bromism would have to be considered as the cause of the elevated chloride result.

Answers to Questions to Consider

The chloride result is in the high abnormal range (See Chloride on CD-ROM), and seems unusual since the other electrolyte results are within the normal range. The glucose result is elevated, but not strikingly so.
These data can be used to calculate the serum osmolality (p.267) and the anion gap (See p 446 and Method Anion Gap in CD-ROM).
The anion gap can be calculated from these results as: $$\begin{split} \text{Anion gap} &= [Na^{+}] - [Cl^{-}] - [CO_{2}] \\ &= 139 - 118 - 20 \\ &= +1 \end{split}$$
The causes of a low anion gap are listed in the Anion Gap chapter in the Methods CD-ROM. In addition to those listed, lipemia can often cause a positive interference in the ferric thiocyanate procedure for chloride (See Method Chloride in CD-ROM).
Yes! A low anion gap has been suggested as a quality control check on electrolyte results (See Method Anion Gap on CD-ROM). Although there are physiological causes of low anion gaps, these are uncommon. Very low anion gaps should therefore suggest to the technologist that an analytical error may be present. The most frequent analytical cause of a low anion gap is a falsely elevated chloride result.
Lipemia of the sample is a common source of interference. In addition to lipemia, the presence of bromide ions (bromism) is an important source of positive interference. Present in many drugs, bromide causes a positive interference not only by reacting in the ferric thiocyanate reaction, but by producing more color on a molar basis than do chloride ions (See Method Chloride on CD-ROM).
The chloridometer would be a good alternative. Although bromism still causes a positive interference, it does so on an equal molar basis. The interference by lipemia is minimal in the chloridometer method.