ACID BASE

Question 1

Abnormal base excess with normal anion gap

Type

Examples x4

Answer 1

normal anion gap metabolic acidosis

(e.g. 
acetazolide, 
hypercholoremia, 
GI losses of HCO3, 
renal tubular acidosis)

Question 2

Normal base excess with abnormal anion gap

Answer 2

lactic acidosis 
(or other high anion gap metabolic acidosis) with pre-existing metabolic alkalosis

HAGMA masked by hypoalbuminemia
(if anion gap is uncorrected)

salicylate toxicity –
respiratory alkalosis plus in increased anion gap metabolic acidosis

Question 3

Anion Gap calculation

Answer 3

(Na + K) - (Cl+HCO3)

Or

Na - (Cl+HCO3)

Question 4

Normal anion gap depends on

Answer 4

Serum phosphate and serum albumin concentrations

An elevated anion gap strongly suggests the presence of a metabolic acidosis

Question 5

Normal anion gap value

Answer 5

varies with different assays, but is typically 4 to 12mmol/L
(if measured by ion selective electrode;

8 to 16 if measured by older technique of flame photometry)

Question 6

If AG > 30 mmol/L

Answer 6

then metabolic acidosis invariably present

If AG 20-29mmol/L then 1/3 will not have a metabolic acidosis

Question 7

ALBUMIN AND PHOSPHATE

formula

why is albumin relevant

How will albumin affect

Answer 7

the normal anion gap depends on serum phosphate and serum albumin
the normal AG = 0.2 x [albumin] (g/L) + 1.5 x [phosphate] (mmol/L)

albumin is the major unmeasured anion and contributes almost the whole of the value of the anion gap.

every 1g/L decrease in albumin will decrease anion gap by 0.25 mmoles

a normally high anion gap acidosis in a patient with hypoalbuminaemia may appear as a normal anion gap acidosis.

this is particularly relevant in ICU patients where lower albumin levels are common

Question 8

HIGH ANION GAP METABOLIC ACIDOSIS (HAGMA)

how does it occur

Causes (broad)

Answer 8

HAGMA results from accumulation of organic acids or impaired H+ excretion

Lactate
Toxins
Ketones
Renal

Question 9

Causes more specific

Answer 9

Causes (CATMUDPILES)

CO, CN
Alcoholic ketoacidosis and starvation ketoacidosis
Toluene
Metformin, Methanol
Uremia
DKA
Pyroglutamic acidosis, paracetamol, phenformin, propylene glycol, paraladehyde
Iron, Isoniazid
Lactic acidosis
Ethylene glycol
Salicylates

Question 10

How to correct for albumin in AG

Answer 10

AG, the corrected AG can be used which is

AG + (0.25 X (40-albumin) expressed in g/L

Question 11

Lab tests to consider include in AG

Answer 11

lactate, glucose, creatinine and urea, urinary ketones, serum levels of methanol, ethanol, paracetamol, salicylates and ethylene glycol

Question 12

NAGMA

overall

causes

Answer 12

NORMAL ANION GAP METABOLIC ACIDOSIS

NAGMA results from loss of HCO3- from ECF

Causes (CAGE)

Chloride excess
Acetazolamide/Addisons
GI causes – diarrhea/vomiting, fistulae (pancreatic, ureters, billary, small bowel, ileostomy)
Extra – RTA

Question 13

NAGMA Causes

Answer 13

Causes (ABCD)

Addisons (adrenal insufficiency)
Bicarbonate loss (GI or Renal)
Chloride excess
Diuretics (Acetazolamide)

Question 14

How to differentiate between GI and Renal causes of NAGMA

Answer 14

Calculate the urinary anion gap to differentiate between a GI and renal cause of a normal anion gap acidosis

urinary anion gap = (Na+ + K+) – Cl-
The remaining significant unmeasured ions are NH4+ and HCO3-
renal causes increased urinary HCO3- excretion thus increased urinary AG
GI causes increased NH4+ excretion thus decreased urinary AG

Question 15

LOW ANION GAP

Answer 15

Causes

Non random analytical errors (increased Na+, increased viscosity, iodide ingestion, increased lipids)

Decrease in unmeasured anions (albumin, dilution)

Increase in unmeasured cations (multimyeloma (cationic IgG paraprotein), hypercalcaemia, hypermagnesaemia, lithium OD, polymixin B)

bromide OD (causes falsely elevated chloride measurements)

Question 16

ADVANTAGES OF THE ANION GAP

Answer 16

Advantages

simple to measure and evaluate acid-base disturbance

can be done at bed side

Question 17

DISADVANTAGES OF THE ANION GAP

Answer 17

Disadvantages

reduced anions (ie hypoalbuminaemia) which is common in critical illness will reduce AG and may mask an elevated AG

unmeasured cations such as lithium and hyperglobulinaemia will reduce AG

hypercalcaemia and hypermagnesemia will reduce the AG

calculation of AG involves the measurement of electrolyte therefore is depends on the accuracy of measurement process.