Anion Gap

Question 1

Anion gap formula and normal value:

Answer 1

Normal AG = 12
AG = Na⁺ - (HCO₃^- + Cl^-)

Question 2

Plasma electroneutrality:

Answer 2

• Blood plasma will always be electrical neutral.
• [cations] = [anions]
• governed/maintained by kidneys.

Question 3

Predominant plasma cation and normal range:

Answer 3

Na+ (136-145 meq/l)

Question 4

Predominant plasma anions and normal ranges:

Answer 4

• Cl- (98-106 meq/l)
• HCO3- (21-30 meq/l)

Question 5

Major unmeasured anion:

Answer 5

albumin

Question 6

Major unmeasured cations:

Answer 6

Mg2+, Ca2+, K+

Question 7

An elevated anion gap (>12) always strongly suggests the presence of:

Answer 7

a metabolic acidosis.

Question 8

What ion fluctuation is common with normal anion gap metabolic acidosis?

Answer 8

• hyperchloremia
• Cl- replaces the HCO3- which is lost

Question 9

What occurs in metabolic acidosis w/ normal anion gap?

Answer 9

• Cl- (measured anion) increases to replace lost HCO3-.
• hyperchloremic acidosis.

Question 10

What occurs in metabolic acidosis w/ increased anion gap?

Answer 10

• Fixed acid (unmeasured anion) increases to replace lost HCO3-.
• Results from metabolism of lactate, ethanol, methanol, ethylene glycol.

Question 11

Why does the anion gap increase if a fixed acid (unmeasured anion) increases to replace lost HCO3-?

Answer 11

1. Bicarbonate levels decrease due to production of fixed acid.
2. Fixed acid continues to be produced and has negative charge.
3. Kidneys excrete Cl- to counter increase in fixed acid.
4. Anion gap increases (AG = Na - (Cl + HCO3))

Question 12

Urine anion gap is the difference between:

Answer 12

• excreted urine cations and excreted urine anions.

Question 13

The urine anion gap is only useful in the setting of:

Answer 13

normal anion gap metabolic acidosis.

NOT USEFUL IN INCREASED ANION GAP METABOLIC ACIDOSIS

Question 14

Urine anion gap equation:

Answer 14

Urine AG = U_Na+ + U_K+ - U_Cl- ≈ 0

Question 15

Urine anion gap value in metabolic acidosis + normal AG + intact renal function:

Answer 15

• UAG < 0
• kidneys form NH4Cl.

Question 16

Steps in formation of a negative urine anion gap value in metabolic acidosis + normal AG + intact renal function:

Answer 16

1. kidneys use ammonia (NH3) to buffer proton.
2. ammonium ion (NH4+) produced.
3. NH4+ pairs with Cl- to form NH4Cl.
4. NH4Cl excreted, increased urine Cl-.

Question 17

Most common cause of metabolic acidosis + normal AG + intact renal function (i.e. UAG < 0):

Answer 17

diarrhea

Question 18

Urine anion gap value in metabolic acidosis + normal AG + impaired renal function:

Answer 18

• UAG > 0
• kidneys cannot form NH4Cl.

Question 19

Reason for positive urine anion gap value in metabolic acidosis + normal AG + intact renal function:

Answer 19

• kidneys cannot buffer H+ with NH3.
• less NH4Cl excreted in urine.
• less negative charge in urine.
• UAG > zero.

Question 20

The only time a ∆-∆ difference should be calculated:

Answer 20

metabolic acidosis with abnormal anion gap

Question 21

The only time a UAG should be calculated:

Answer 21

metabolic acidosis with normal anion gap

Question 22

∆-∆ equation:

Answer 22

∆-∆ = ∆AG - ∆HCO3-

Question 23

If ∆-∆ = 0 ± 5 in the setting of metablolic acidosis with abnormal anion gap, what is the diagnosis?

Answer 23

AG metabolic acidosis only

Question 24

If ∆-∆ > +5 in the setting of metablolic acidosis with abnormal anion gap, what is the diagnosis?

Answer 24

• evaluate for AG metabolic acidosis + metabolic alkalosis

Question 25

If ∆-∆ < -5 in the setting of metablolic acidosis with abnormal anion gap, what is the diagnosis?

Answer 25

• evaluate for AG metabolic acidosis + non-AG metabolic acidosis