2.8: Electrolytes/anion Gap
- Page ID
- 38636
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)RELATED READING: Chapter 24
See Methods in CD-ROM for Anion Gap, Chloride, Carbon Dioxide, Sodium and Potassium
OBJECTIVES
Following completion of this exercise, appropriate discussion, and related reading, the student will be able to:
- Determine Na+, K+, and Cl- levels in serum samples with 90% accuracy.
- Given the CO2 values for the samples tested, calculate the anion gap on these samples using the measured values.
- List the causes of an increased and a decreased anion gap.
PRINCIPLE
Routine “electrolytes” usually consist of Na+, K+, Cl-, and HCO3-(CO2) determinations. This allows for the calculation of an anion gap. This value is used to evaluate the relationship between anions and cations. There are several ways this calculation can be performed. Two of the more common methods are:
Reference Intervals
| Anion Gap = (Na+, K+) - (Cl- + HCO3-) | 11-18 mmol/L |
| Anion Gap = Na+ - (Cl- + HCO3-) | 7-14 mmol/L |
The reference intervals are instrument specific. The bias between instruments will determine the reference interval for an institution.
We have used a “contamination” exercise for chlorides similar to the one suggested for flame photometry. Care should be exercised in using the acid reagent.
*Acid reagent can be prepared from stock reagents in the following proportions:
- 900 mL deionized water
- 6.4 mL conc. nitric acid
- 100 mL conc. glacial acetic acid
MATERIALS
Serum Samples Instruments may be any of these:
- Controls
- Flame photometer, Chloridometer
- Na/K Standard
- ISE chemistry instrument
- Cl Standard
- Blood gas instrument
- Acid Buffer
- Point of care device
- Sample cups
- Lithium Diluent if flame
PROCEDURE
Perform Na+, K+, and Cl- (and CO2 if available) determinations on the serum samples provided. Using the CO2 values provided by your instructor, determine the Anion Gap for each sample. For the samples with abnormal Anion Gap values, consult with your instructor to determine what other test results are known.
| DATA SHEET, EXERCISE #8 |
NAME: ___________ DATE: ___________ |
RESULTS:
| Na+ | K+ | Cl- | CO2 | ANION GAP | |
|---|---|---|---|---|---|
| Analyte Concentrations, mmol/L | |||||
| Normal Control | |||||
| Abnormal Control | |||||
| Sample # | |||||
| Sample # | |||||
| Sample # | |||||
| Sample # | |||||
| Sample # | |||||
| Sample # | |||||
Discussion Questions
- What are two common diseases or conditions that cause an elevated anion gap?
- Which of the additional test results provided by your instructor would have a direct affect on the anion gap.
- Which of the additional test results are associated with diseases that produce abnormal anion gaps?
- An abnormally low anion gap has been suggested as a quality control indicator for false laboratory results. For which analyte? What is the interferent? By which method? Why is a low anion gap a useful indicator?