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Sodium, water and potassium

17 min

Skills you'll build4 skills
  • Use measured osmolality to tell hypotonic hyponatremia from pseudohyponatremia
  • Calculate an osmolal gap using the stated formula and units
  • Distinguish potassium redistribution from a change in total-body stores
  • Decide whether to release or recollect a hemolyzed potassium specimen

At the bench: Assess specimen interference before reporting a chemistry result · Review electrolyte results and related calculations for consistency

Try first

Try first

A routine panel shows sodium 129 mmol/L and glucose 98 mg/dL. No osmolality was measured. The ward asks whether the patient has excess body water. Which result do you suggest they add first?

Keep going to see the reasoning.

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Sodium and water balance

Serum sodium mostly reflects water balance. Sodium and its anions supply about 90% of plasma osmolality, which the body maintains between 275 and 295 mOsm/kg.1 A low sodium is classified by the measured serum osmolality:1

Measured osmolalityPatternCause
275–295 mOsm/kgConsider pseudohyponatremiaLipid or protein lowers sodium read by an indirect electrode. Other possibilities include retained mannitol or glycine, or urea or ethanol raising the osmolality of a truly hypotonic serum5
Above 295 mOsm/kgHypertonicGlucose or mannitol pulls water out of cells
Below 275 mOsm/kgHypotonicTrue excess of water relative to sodium

With high glucose, a corrected sodium shows how much of the low value is water shift:

Corrected sodium (mmol/L) = measured sodium + 1.6 × [(glucose in mg/dL − 100) ÷ 100]

Measured data suggest a larger factor, near 2.4, so the procedure states which factor is used.2 The laboratory reports the measured sodium.

The osmolal gap

The panel predicts osmolality from sodium, glucose and urea. Calculated osmolality (mOsm/kg) = 2 × sodium (mmol/L) + glucose (mg/dL) ÷ 18 + blood urea nitrogen (mg/dL) ÷ 2.8. Osmolal gap = measured − calculated osmolality, read against the interval for the formula used.1 A raised gap points to an unmeasured neutral solute, such as ethanol, methanol, ethylene glycol or mannitol.1

Potassium shifts and body stores

About 98% of body potassium is inside cells, so serum reflects only a small extracellular fraction.3 Insulin lack, hyperosmolality and mineral acidosis move potassium out of cells. In diabetic ketoacidosis the serum potassium runs high, and total-body potassium is depleted. Insulin moves potassium back into cells and unmasks the deficit.1,3

Potassium in a hemolyzed specimen

Red cells contain far more potassium than plasma. Lysed cells raise the result by an amount that depends on the analyzer, the method and the patient's cells. No fixed correction factor gives the true value.4 Above the laboratory's hemolysis limit for potassium, the result is not reported and a careful new specimen is requested. A valid critical result on a good specimen is called at once.1,4

Follow the bench demonstration

Sources for this step
References
  1. Rifai N, Chiu RWK, Young I, Burnham CAD, Wittwer CT, eds. Tietz Textbook of Laboratory Medicine. 7th ed. Elsevier; 2023.
  2. Hillier TA, Abbott RD, Barrett EJ. Hyponatremia: evaluating the correction factor for hyperglycemia. Am J Med. 1999;106(4):399-403. doi:10.1016/S0002-9343(99)00055-8
  3. Palmer BF, Clegg DJ. Physiology and pathophysiology of potassium homeostasis. Adv Physiol Educ. 2016;40(4):480-490. doi:10.1152/advan.00121.2016
  4. Simundic AM, Baird G, Cadamuro J, Costelloe SJ, Lippi G. Managing hemolyzed samples in clinical laboratories. Crit Rev Clin Lab Sci. 2020;57(1):1-21. doi:10.1080/10408363.2019.1664391
  5. Spasovski G, Vanholder R, Allolio B, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Eur J Endocrinol. 2014;170(3):G1-G47. doi:10.1530/EJE-13-1020

Watch one

Read each decision and its explanation.

An emergency department patient's specimen gives these results. The laboratory's procedure uses the correction factor 1.6.

Which pattern explains the low sodium?

TestResultReference intervalFlag
Sodium128 mmol/L Low135–145 mmol/L135–145Low
Glucose720 mg/dL High70–99 mg/dL70–99High
BUN14 mg/dL8–24 mg/dL8–24
Osmolality, measured305 mOsm/kg High275–295 mOsm/kg275–295High

Specimen: H 5, L 10, I 1. Serum, no collection problems recorded

  1. Check the osmolality. At 305 mOsm/kg it is above 295 mOsm/kg, so the serum is hypertonic.

    Measured osmolality is the first step in classifying a low sodium.

  2. Identify the solute. Glucose is 720 mg/dL.

    A hypertonic low sodium needs a solute that stays outside cells and pulls water out.

  3. Calculate: 2 × 128 + 720 ÷ 18 + 14 ÷ 2.8 = 256 + 40 + 5 = 301 mOsm/kg, a gap of 4 mOsm/kg.

    The calculated osmolality shows whether glucose accounts for the measured value.

  4. Correct the sodium: 128 + 1.6 × (620 ÷ 100) = 128 + 9.9 = 137.9 mmol/L.

    The corrected sodium estimates the sodium without the water drawn out of cells.

  5. Report the measured sodium of 128 mmol/L.

    The patient record contains measured values. The corrected value is an aid to interpretation.

This is hypertonic, translocational hyponatremia from glucose. The corrected sodium is 137.9 mmol/L, and the measured 128 mmol/L is reported.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

Measured osmolality is 310 mOsm/kg, sodium 140 mmol/L, glucose 90 mg/dL, and blood urea nitrogen (BUN) 14 mg/dL. Using 2 × Na + glucose/18 + BUN/2.8, what is the osmolal gap?

Incorrect. This subtracts measured from calculated osmolality. The gap is measured minus calculated: 310 − 290 = 20 mOsm/kg.

Incorrect. That is the calculated osmolality. The gap subtracts it from the measured 310 mOsm/kg.

Correct. Calculated osmolality = 2 × 140 + 90 ÷ 18 + 14 ÷ 2.8 = 280 + 5 + 5 = 290 mOsm/kg, and the gap is 310 − 290 = 20 mOsm/kg.

Hint
  1. Calculate osmolality first: double the sodium, then add glucose ÷ 18 and BUN ÷ 2.8.
  2. The gap is measured minus calculated.

Review Calculated osmolality and the osmolal gap

Problem 2 of 3 · Hints available

A patient in diabetic ketoacidosis has potassium 5.9 mmol/L on a specimen with no hemolysis. The care team asks whether total-body potassium is high. Which answer fits?

Serum contains about 2% of body potassium. Insulin lack and acidosis move potassium out of cells, so serum runs high. Total-body stores are usually depleted.

Read a high serum potassium as excess body potassium

Insulin deficiency and acidosis move potassium out of cells, so in diabetic ketoacidosis serum potassium runs high with total-body potassium depleted. Reading the high value as excess stores misses the deficit that insulin therapy reveals as potassium moves back into cells.

Insulin lack and acidosis move potassium from cells into the serum. Urine losses deplete the stores, and insulin unmasks the deficit as potassium moves back into cells.

This specimen shows no hemolysis. The high value is a real change in the patient's serum.

Hint
  1. Start with where most of the body's potassium is located.
  2. Consider how insulin lack and acidosis move that potassium.

Review Potassium

Problem 3 of 3 · On your own

A clear serum gives sodium 126 mmol/L, glucose 95 mg/dL and measured osmolality 262 mOsm/kg. Which pattern fits?

Pseudohyponatremia keeps measured osmolality inside 275 to 295 mOsm/kg. This osmolality is 262 mOsm/kg.

Read a low sodium as hypotonic without measured osmolality

A low sodium with normal measured osmolality can reflect pseudohyponatremia from lipid or protein. Urea or ethanol can also raise measured osmolality in true hypotonic hyponatremia. A high osmolality can reflect glucose or mannitol translocation. Treating every low sodium as hypotonic skips that step and can report an analytical artifact or an osmotic water shift as true water excess.

Translocation needs an osmolality above 295 mOsm/kg and a solute such as high glucose. Glucose here is 95 mg/dL.

Measured osmolality of 262 mOsm/kg is below 275 mOsm/kg, so the serum is hypotonic. Urine osmolality and urine sodium are the next tests for classifying its cause.

Review Hyponatremia

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • Tobias Staines, 23, MRN 6029481, arrives in the emergency department with vomiting and fast breathing.
  • His first specimen came from a difficult syringe draw.
  • Your procedure does not report potassium above a hemolysis index of 50.
  • Potassium is critical at or above 6.2 mmol/L.
  • QC for the run is acceptable.
TestResultReference intervalFlag
Potassium6.9 mmol/L High3.5–5.1 mmol/L3.5–5.1High
Glucose540 mg/dL High70–99 mg/dL70–99High
Total CO28 mmol/L Critical22–29 mmol/L22–29Critical

Specimen: H 180, L 8, I 1. Serum, difficult syringe draw

Decision 1 of 3

What do you do with the potassium of 6.9 mmol/L?

The hemolysis index of 180 is more than three times the potassium limit. The value includes potassium released from lysed red cells and does not reflect the patient's serum potassium.

Read a hemolyzed tube as hemolysis in the patient

Forceful aspiration, a narrow needle, or vigorous mixing lyses red cells in the tube and raises plasma hemoglobin, potassium, and LDH. This is in vitro hemolysis. Reporting it as hemolysis in the patient starts a workup on an artifact. Haptoglobin, bilirubin, and a reticulocyte count on a properly collected specimen show whether the patient has in vivo hemolysis.

The potassium that hemolysis adds depends on the analyzer, the method and the cells. A corrected number is an estimate presented as a measurement.

Corrected potassium with a fixed hemolysis factor

The potassium that hemolysis releases depends on the analyzer, the method, and the patient's cells, so no fixed factor per hemolysis-index unit recovers the true value. Reporting a corrected potassium presents an estimate as a measurement. Above the laboratory's hemolysis reporting limit, the specimen is recollected.

Above the limit the potassium is not reported. The redraw gives a value that reflects the patient, and the care team knows why the result is delayed.

Review Preanalytic artifact in potassium results

Decision 2 of 3

The redraw has a hemolysis index of 10. Its potassium is 6.3 mmol/L. What do you do?

The first specimen's problem came from the draw. This specimen is below the hemolysis limit, and delaying a critical result puts the patient at risk.

The hemolysis index of 10 is well below the limit of 50. At 6.3 mmol/L the potassium meets the critical limit and is called at once, with read-back.

An index of 10 is far below the potassium limit. A hemolysis comment would cast doubt on a valid critical result.

Review Preanalytic artifact in potassium results

Decision 3 of 3

The physician asks whether the high potassium means his body stores are high. Which answer fits these results?

Insulin lack and acidosis move potassium out of cells. Vomiting and urine losses deplete the stores, and insulin treatment uncovers the deficit.

The glucose of 540 mg/dL and total CO2 of 8 mmol/L fit insulin lack with acidosis. Both move potassium out of cells, so serum runs high. The stores are usually depleted.

Read a high serum potassium as excess body potassium

Insulin deficiency and acidosis move potassium out of cells, so in diabetic ketoacidosis serum potassium runs high with total-body potassium depleted. Reading the high value as excess stores misses the deficit that insulin therapy reveals as potassium moves back into cells.

The redraw is a valid specimen with a hemolysis index of 10, and its result can be interpreted as reported.

Review Potassium

For each specimen, the hemolysis index was compared with the potassium limit of 50:

  • At an index of 180 the first potassium includes potassium from lysed cells, so it is not reported and a new specimen is requested.
  • At an index of 10 the redraw is acceptable, so its critical potassium is called at once.

For reference

Sources checked