The body certainly strives to achieve balance and equilibrium. When extra H ions or acids are released, the body experiences a condition called metabolic acidosis. This condition speeds up your breathing rate and lowers your plasma level. The anion difference is used to determine the exact cause of this condition. This value calculates the unmeasured anions, namely phosphate, sulfate, and protein in the plasma. Calculating the anion difference is very easy using the standard formula. To get started, see Step 1 below.
Step
Part 1 of 2: Calculating Your Anion Difference
Step 1. Determine your sodium (Na⁺) level
The normal range for sodium is 135 – 145 mEq/L. It is important to know the sodium level in your body. You can check your sodium level through a blood test that can be done by your doctor.
Step 2. Determine your potassium (K⁺) level if necessary
The normal range of potassium is 3.5 – 5.0 mEq/L. However, there is a different formula that doesn't require you to use potassium levels. This is because K⁺ is found in such small quantities in plasma that it often does not affect calculations.
Since there are formulas that don't require a potassium level, you can skip this step
Step 3. Determine your chloride (Cl⁻) level
The normal range for chloride is 97 – 107 mEq/L. Your doctor will also examine it.
Step 4. Determine your bicarbonate (HCO₃⁻) level
The normal range for bicarbonate is 22 – 26 mEq/L. Again, this level is determined through the same series of tests.
Step 5. Find the normal reference value of the anion difference
The normal value of the anion difference is 8 – 12 mEq/L without potassium. However, if potassium is used, the normal range value will change to 12 – 16 mEq/L.
- Note that all of these electrolyte levels can be determined through a blood test.
- Pregnant women may also have different levels. We will discuss this in the next section.
Step 6. Use the standard formula to calculate the anion difference
There are 2 formulas you can use to calculate the anion difference:
- First formula: Anion difference = Na⁺ + K⁺ – (Cl⁻ + HCO₃⁻). This formula can be used if we have a potassium value. However, the second equation is used more often than the first equation.
- Second formula: Anion difference = Na⁺ – (Cl⁻ + HCO₃⁻). You can see that potassium is omitted in this second equation. This formula is used more often than the first formula. However, you can use both formulas depending on your preference.
Step 7. Know what a healthy outcome looks like
Again, normal values are 8 – 12 mEq/L without potassium and 12 – 16 mEq/L with potassium. Here are two examples:
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Example 1: Na⁺ = 140, Cl⁻= 100, HCO₃⁻= 23
AG= 140 – (98 + 23)
AG = 24
The anion difference is 24. Thus, the person is positive for metabolic acidosis
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Example 2: Na⁺ = 135, Cl⁻= 100, HCO₃⁻= 25
AG= 135 – (100 + 25)
AG = 10
The anion difference is 10. Thus, the results are normal and the person does not have metabolic acidosis. The results are within the normal range of 8 – 12 mEq/L
Part 2 of 2: Understanding Anion Difference
Step 1. Know the meaning of anion difference
Anion difference measures the difference between sodium and potassium cations and chloride and bicarbonate anions in patients who have kidney problems or altered mental status – in other words, your pH balance. This value represents the concentration of unmeasured anions in the plasma, such as proteins, phosphates, and sulfates. This is a term that indicates that your body produces the right ingredients, but at inappropriate levels.
Determining the value of the anion difference is very important in arterial blood gas (AGD) analysis. The basic concept is that the net charge of cations and anions must be equal in order to achieve balance in your body
Step 2. Understand the importance of the anion difference
This value is basically a measurement for patients with kidney or digestive problems. This test certainly does not show one particular condition. However, this test can figure out a few things and narrow down the areas that need attention.
- The anion difference can be used to detect metabolic acidosis in which the pH level in your body is out of balance. This value differentiates causes of metabolic acidosis and helps confirm other results. Ask your doctor to help you understand this process.
- Suppose a patient has lactic acidosis (where there is also a buildup of lactate). In this case, the serum bicarbonate level will automatically decrease (due to lactate buildup) so when you calculate the anion difference, you will see that the anion difference is increasing.
Step 3. Know what will happen during the test
A sample of the serum anion difference is taken from your vein using a serum separator tube. Here's how it'll go:
- A medical scientist or medical technologist extracts blood from a vein, possibly in your arm.
- He or she may ask if you have a history of allergies to latex. If you are allergic, they will use other ingredients to make sure you don't have an allergic reaction.
- Tell them about any medical conditions or medications that can cause excessive bleeding or if you have psychological problems associated with sharp objects such as needles.
- Your specimen will be stored in a special refrigerator (biorefrigerator) and lined up for inspection. When everything is done, your doctor will contact you to discuss the results.
Step 4. Know how to interpret your results
Your doctor will relate the diagnosis to how you look, how you feel, and the symptoms you report. Once the results are known and confirmed, your doctor will explain to you the next steps. If your doctor feels that the results may be wrong, he or she may order another test to confirm the results.
- Decreased anion gap can be associated with a variety of conditions such as hypoalbuminemia and bromide poisoning. Normal results are expected when a patient is recovering from diabetic ketoacidosis or recovering from bicarbonate deficiency due to prolonged diarrhea.
- An increase in the anion gap may indicate lactic acidosis or renal failure. Interpretation of the results may vary, depending on various factors and the main condition experienced by the patient.
- The "normal" anion difference for pregnant women is slightly different. During the first three months, the normal anion difference ranges from 10 to 20 mmol/L. During the second and third months, the normal value decreases from 10 to 11 to a maximum of 18 mmol/L.
Step 5. Note that there are other things that can affect the value of the anion difference
Errors in data collection can occur and affect your laboratory results. Time, dilution, and sample size are critical to getting accurate results. Delays in processing the collected specimens and exposure to air over long periods of time can also lead to increased bicarbonate levels. If this is the case, the anion gap may decrease by about 2.5 mEq/L for every gram/dL of albumin concentration that is removed from your blood. Your doctor should be able to deal with this (besides avoiding it completely).
