MLAB 2401: CLINICAL CHEMISTRY
KERI BROPHY-MARTINEZ
ABG Interpretation
EVALUATING ACID-BASE DISORDERS

ARMADA

acid-base balance made easy in 4 steps
1. A cidosis or Alkalosis?
2. R espiratory disorder?
acidosis or alkalosis

check pCO2
M etabolic disorder?

acidosis or alkalosis
 check HCO3

The one that matches the pH (acidosis or alkalosis), is the
primary disorder.
3. A nion Gap?
D elta AG?
4. A ssess compensation
STEP 1: EVALUATE PH
 Evaluate
pH
 < 7.35 = Acidosis
 > 7.45 = Alkalosis
STEP 2: THINK “ROME”

To determine whether the disorder is respiratory or
metabolic use “ROME”
R = Respiratory
 Seesawing (opposite)
 is pCO2 less than 35 (alkalosis) or more than 45 (acidosis)
O = Opposite [pH & pCO2]
M = Metabolic
 Swinging together
o is HCO3 less than 22 (acidosis) or more than 26 (alkalosis)
E = Equal [pH & HCO3]
STEP 3: ANION GAP (AG) & DELTA AG

Examine AG and Delta AG
Anion gap = (Na + K) - (Cl + [HCO3])
(all units mmol/L)
 Since K is a small number, then . . .

AG = Na+ - (Cl- + HCO3-)

Delta AG = patient's AG - 12 mEq/L
MORE ON THE ANION GAP

MUDPILES
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Methanol
Uremia of renal failure
Diabetes or ketoacidosis
Paraldehyde toxicity
Isoniazid
Lactic acidosis
Ethylene glycol
Salicylate
STEP 4: ASSESS COMPENSATION
 To
assess compensation compare
pCO2 and HCO3- to reference range
 Is
the pH normal?
 Yes
 Respiratory disorder
compensated by kidneys
 Metabolic disorder compensated
by lungs
STEP 4: ASSESS COMPENSATION

If the pH is still outside the reference range

Partial compensation has occurred
ACID-BASE - PROBLEM #1
pH = 7.56
pCO2 = 43 mmHG
HCO3 = 38 mEQ/L
pH > 7.45, then alkalosis
pH & HCO3 swinging up, then Metabolic Alkalosis
Since pH still high and pCO2 normal then it is Uncompensated
What are some causes of Metabolic Alkalosis?
ACID-BASE PROBLEM #2
pH = 7.23
PCO2 = 57 mmHG
HCO3 = 23 mEQ/L
pH < 7.35, then acidosis
Bicarb is normal but CO2 is elevated or opposite of pH so this is Respiratory Acidosis
Since the pH is still low and the HCO3 is normal it is uncompensated
What causes Respiratory Acidosis?
ACID-BASE PROBLEM #3
pH = 7.23
pCO2 = 45
HCO3 = 19
pH < 7.35, then acidosis
pCO2 normal but HCO3 is low or swinging w/ pH so it is Metabolic Acidosis
pCO2 normal so uncompensated
What causes Metabolic Acidosis?
ACID-BASE PROBLEM #4
pH = 7.51
pCO2 = 29 mmHG
HCO3 = 20 mEq/L
pH > 7.45, then alkalosis
pCO2 is low or opposite pH so it is Respiratory Alkalosis
Since HCO3 is low it is partially compensated
What causes Respiratory Alkalosis?
How is it compensated?
REFERENCES
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Bishop, M., Fody, E., & Schoeff, l. (2010). Clinical
Chemistry: Techniques, principles, Correlations. Baltimore:
Wolters Kluwer Lippincott Williams & Wilkins.
Carreiro-Lewandowski, E. (2008). Blood Gas Analysis and
Interpretation. Denver, Colorado: Colorado Association for
Continuing Medical Laboratory Education, Inc.
Jarreau, P. (2005). Clinical Laboratory Science Review (3rd
ed.). New Orleans, LA: LSU Health Science Center.
Sunheimer, R., & Graves, L. (2010). Clinical Laboratory
Chemistry. Upper Saddle River: Pearson .
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