Electrolytes and Osmolality

Question 1

Osmolality

Answer 1

Measure of dissolved particles in solution compared to water

Serum ref range = 275-295 mOsm/kg

Question 2

Osmol gap and equations

Answer 2

Difference between measured and calculated osmolality
Should be < 15

2Na + glucose/20 + BUN/3
1.86Na + glucose/18 + BUN/2.8 + 9

Question 3

Major cations

Answer 3

Sodium, potassium, calcium, and magnesium

Question 4

Major anions

Answer 4

Chloride, bicarbonate, phosphate, sulfate, organic acids, protein

Question 5

Sodium

Answer 5

Extra cellular cation, largest part of serum osmolality
Ref range = 136-145 mmol/L
Renal threshold = 110-130 mmol/L

Low from depletion (Addisons, diuretics, vomiting/diarrhea, burns) or dilution (overhydration, CHF, cirrhosis, nephrotic syndrome)
High when water lost (diarrhea, sweating, diabetes) or sodium retained (ingestion, hyperaldosteronism, dialysis)

Question 6

Potassium

Answer 6

Intercellular cation, affected by hemolysis
Ref range = 3.4-5.0 mmol/L

Low from decreased intake, diarrhea/vomiting/laxatives, excess insulin, diuretics
High from increased intake, renal failure, leukemia, chemo, metabolic acidosis

Question 7

Chloride

Answer 7

Extra cellular anion, change with sodium
Ref range = 98-107 mmol/L

Low when vomiting, diuretics, burns, aldosterone deficiency
High from diarrhea, dehydration, renal tubular disease, loss of bicarbonate

Question 8

Bicarbonate

Answer 8

Buffer system of blood
Ref range = 22-29 mmol/L

Low in metabolic acidosis, diabetic keto acidosis, salicylate toxicity
High in metabolic alkalosis, emphysema, severe vomiting

Question 9

Anion gap

Answer 9

Cations minus anions

Na - (Cl + HCO3) = ref range = 7-16 mmol/L

(Na + K) - (Cl + HCO3) = ref range = 10-20 mmol/L

Question 10

Calcium

Answer 10

50% free/ionized, 40% bound to protein, 10% bound to anions
Free form is biologically active

Total calcium = 8.6-10.3 mg/dL
Free calcium = 4.6-5.3 mg/dL

PTH increases serum calcium (break down bone and reabsorb in kidneys)
Vitamin D increases serum calcium (enhanced absorption in intestines)
Calcitonin inhibits other two, decreases serum calcium

Question 11

Phosphorus

Answer 11

PTH increases renal excretion (decrease serum phosphorus)
Vitamin D increases absorption in intestines/kidneys (increases serum phosphorus)

Ref range = 2.5-4.5 mg/dL

Question 12

Magnesium

Answer 12

55% free/ionized, 30% bound to protein, 15% complexed
Free is biologically active

PTH increases serum Mg by absorption at kidneys/intestines
High caused by renal failure, antacids
Low caused by GI disorders, drugs, renal diseases, diabetes, alcoholism

Ref range = 1.7-2.4 mg/dL

Question 13

Iron ref ranges

Answer 13

Serum iron = 45-160 ug/dL
TIBC = 250-425 ug/dL
% saturation = 15-55%

Question 14

Iron deficiency indices

Answer 14

Serum iron decreased
% saturation decreased
TIBC increased

Question 15

Malnutrition iron indices

Answer 15

Serum iron decreased
% saturation varies
TIBC decreased

Question 16

Iron overdose indices

Answer 16

Serum iron increased
% saturation increased
TIBC decreased

Question 17

Hemochromatosis iron indices

Answer 17

Serum iron increased
% saturation increased
TIBC decreased

Question 18

Ferritin

Answer 18

Reflects irons stores, early indicator of iron-deficiency

Male ref range = 20-250 ng/mL
Female ref range = 10-120 ng/mL