[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Clinical skills](https://durgical.com/en/clinical-skills/) › [Reading lab results](https://durgical.com/en/clinical-skills/areas/labs/) › Electrolytes, urea and creatinine

# Electrolytes, urea and creatinine

- Interpretation

A routine blood panel of the main salts, bicarbonate and kidney waste products that shows fluid balance and how well the kidneys filter.

Updated Nov 13, 2026

## Key points

- Read creatinine against the baseline: a rise of 0.3 mg/dL within 48 hours, or to 1.5 times baseline within 7 days, defines acute kidney injury.
- Exclude pseudohyperkalaemia (haemolysis, delay, fist clenching, very high platelets or white cells), but with ECG changes treat without waiting for a repeat.
- Hyponatraemia: serum osmolality first, then volume status at the bedside, then urine osmolality and urine sodium.
- Anion gap = Na − (Cl + HCO3), usually about 8–12 mmol/L; correct it for a low albumin before calling it normal.
- A urea:creatinine (BUN:creatinine) ratio above 20 suggests a prerenal cause or upper gastrointestinal bleeding.

## What it is

The electrolyte and renal panel (often called U&E, or part of the basic metabolic panel) reports sodium, potassium, chloride, bicarbonate, urea (as blood urea nitrogen, BUN, in many laboratories) and creatinine, often with glucose. Sodium mainly reflects water balance rather than total body sodium, so a low value is read against serum osmolality and the patient's volume status. Potassium lives mostly inside cells: small shifts, or a damaged sample, change the serum value, and the danger of a high or low level is its effect on the heart. Chloride and bicarbonate give the anion gap, the first clue to an acid–base problem. Urea and creatinine estimate kidney filtration: creatinine is made by muscle at a fairly steady rate and filtered, so it rises when filtration falls, but it also depends on muscle mass and lags behind an acute injury. Laboratories report an estimated glomerular filtration rate (eGFR) from creatinine, age and sex. The panel is read against the patient, the previous results and the drug chart, never one number at a time.

## Reference values and concepts

- Usual adult ranges (they vary by laboratory): sodium 135–145 mmol/L, potassium 3.5–5.0 mmol/L, chloride 98–106 mmol/L, bicarbonate 22–29 mmol/L.
- Blood urea nitrogen about 7–20 mg/dL; creatinine about 0.6–1.2 mg/dL, lower in women, older adults and people with little muscle.
- Creatinine clearance is about 97–137 mL/min in men and 88–128 mL/min in women; eGFR is reported in mL/min/1.73 m² and assumes a steady state.
- Serum osmolality is about 275–295 mOsm/kg; calculated osmolality ≈ 2 × Na + glucose ÷ 18 + BUN ÷ 2.8 (glucose and BUN in mg/dL).
- Hyponatraemia: mild 130–134, moderate 125–129, severe below 125 mmol/L; symptoms depend as much on the speed of the fall as on the value.
- Potassium above 5.5 mmol/L is hyperkalaemia; a level above about 6.5 mmol/L, or any level with ECG changes, is treated as an emergency.
- In hyperglycaemia, sodium falls about 1.6 mmol/L for each 100 mg/dL of glucose above 100 mg/dL: water shifts out of cells, not a true sodium deficit.

## Reading it step by step

- Check the patient, the sample and when it was taken — haemolysis or a delay before analysis falsely raises potassium.
- Compare with previous results — the trend and the baseline creatinine matter more than one value.
- Read sodium as a water problem: confirm a low serum osmolality, then judge volume status — this splits the causes of hyponatraemia.
- Add urine osmolality and urine sodium — dilute urine points to excess water; urine sodium above 30 mmol/L to SIADH, diuretics or adrenal causes.
- Read an abnormal potassium with an ECG — the heart's response, not the number alone, sets the urgency.
- Calculate the anion gap and correct it for albumin — a high gap points to an added acid such as ketones or lactate.
- Read urea and creatinine together — a high ratio suggests low perfusion or GI bleeding; a parallel rise suggests kidney injury.
- Classify acute kidney injury as prerenal, renal or postrenal, with urine output and a urinalysis — the class decides the next test.
- Review the drug chart (diuretics, ACE inhibitors, NSAIDs, potassium supplements) — many abnormalities are caused by treatment.

## Common patterns and where they point

| Pattern | Think of | Check next |
| --- | --- | --- |
| Low sodium, low osmolality, dry patient | Vomiting, diarrhoea, diuretics, adrenal insufficiency | Urine sodium (low in losses outside the kidney), cortisol |
| Low sodium, euvolaemic, concentrated urine | SIADH, hypothyroidism, adrenal insufficiency, drugs | Urine osmolality and sodium, TSH, cortisol |
| Low sodium with oedema | Heart failure, cirrhosis, nephrotic syndrome | Examination, albumin, urine protein |
| High potassium, normal ECG, haemolysed sample | Pseudohyperkalaemia | Repeat without haemolysis, analysed promptly |
| High potassium with high creatinine | Acute or chronic kidney injury, ACE inhibitors, potassium-sparing diuretics | ECG, drug chart, urine output, blood gas |
| Urea:creatinine ratio above 20 | Prerenal state, upper GI bleeding, high protein load, steroids | Volume status, haemoglobin, stool colour |
| Low bicarbonate, high anion gap | Ketoacidosis, lactic acidosis, kidney failure, toxins | Blood gas, lactate, ketones, osmolar gap |
| Low bicarbonate, normal gap, high chloride | Diarrhoea, renal tubular acidosis, large volumes of saline | History, urine anion gap |

## Patterns and common mistakes

- Pseudohyperkalaemia: haemolysed sample, long tourniquet and fist clenching, delayed processing, or very high platelet or white cell counts; repeat if the ECG is normal.
- Pseudohyponatraemia with very high lipids or proteins on some analysers (normal osmolality), and hypertonic hyponatraemia from hyperglycaemia or mannitol (high osmolality).
- Correcting chronic hyponatraemia too fast risks osmotic demyelination; many guidelines limit the rise to about 8–10 mmol/L in 24 hours.
- Creatinine lags behind a fall in filtration and is low when muscle mass is low, so a 'normal' value can hide poor kidney function in frail patients.
- eGFR equations are unreliable in acute kidney injury, pregnancy, amputation and extremes of body size; they need a steady creatinine.
- Hypokalaemia with metabolic alkalosis suggests vomiting or diuretics; with metabolic acidosis, diarrhoea or renal tubular acidosis.
- A low anion gap is usually a low albumin; also consider paraproteins (myeloma) or lithium.

## Next step

- High potassium with ECG changes, or very high: call a senior now, monitor the heart and start emergency treatment (calcium to protect the heart, insulin with glucose or salbutamol to shift potassium, then removal).
- Severe or symptomatic hyponatraemia (seizures, confusion, reduced consciousness): urgent senior and high-dependency review; correct under close monitoring of sodium.
- Acute kidney injury: hold nephrotoxic drugs, assess volume, exclude obstruction with a bladder scan or ultrasound, and repeat creatinine and potassium.
- High anion gap: measure lactate, ketones and glucose; if unexplained, an osmolar gap and salicylate level (see the blood gas cases).
- Repeat abnormal values after each intervention and record the trend; a low eGFR for more than 3 months with urine albumin stages chronic kidney disease.

## Test yourself

Think of the answer, then tap the question to see it.

## A well patient has potassium 6.4 mmol/L, a normal ECG and a sample marked haemolysed. What is the next step?

Repeat a non-haemolysed sample analysed promptly, without fist clenching or a long tourniquet; haemolysis releases potassium from red cells. With ECG changes, treat first.

## Case: sodium 124 mmol/L, serum osmolality 260 mOsm/kg, euvolaemic, urine osmolality 450 mOsm/kg, urine sodium 50 mmol/L. What is the likely mechanism?

Hypotonic euvolaemic hyponatraemia with inappropriately concentrated urine and high urine sodium: SIADH, once hypothyroidism, adrenal insufficiency and diuretics are excluded.

## Sodium 140, chloride 104 and bicarbonate 12 mmol/L, albumin 4 g/dL. What is the anion gap and what does it mean?

140 − (104 + 12) = 24 mmol/L, a high anion gap: an added acid such as ketoacids or lactate, kidney failure or a toxin.

## Why can a 'normal' creatinine mislead in a frail older patient?

Creatinine reflects muscle mass as well as filtration; with little muscle it stays in range despite a low filtration rate, so estimate GFR or clearance and compare with the baseline.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/clinical-skills/electrolytes-urea-and-creatinine/)

## Elsewhere in DurgiPedia

- [Sodium and extracellular volume regulation (with natriuretic peptides)](https://durgical.com/en/physiology/sodium-and-extracellular-volume-regulation/)
- [Potassium balance](https://durgical.com/en/physiology/potassium-balance/)
- [Renal clearance](https://durgical.com/en/physiology/renal-clearance/)
- [The ECG in potassium and calcium disorders](https://durgical.com/en/physiology/the-ecg-in-potassium-and-calcium-disorders/)
- [Anion gap and delta ratio](https://durgical.com/en/biochemistry/topics/anion-gap-and-delta-ratio/)
- [Creatinine and estimated glomerular filtration rate (eGFR)](https://durgical.com/en/biochemistry/topics/creatinine-and-estimated-glomerular-filtration-rate/)

## Connections

## Related 5

- [Arterial blood gas: worked cases for the station](https://durgical.com/en/clinical-skills/arterial-blood-gas-worked-cases-for-the-station/)
- [Reading a urinalysis](https://durgical.com/en/clinical-skills/reading-a-urinalysis/)
- [Hyperkalaemia and other electrolyte changes](https://durgical.com/en/clinical-skills/hyperkalaemia-and-other-electrolyte-changes/)
- [Reading a full blood count](https://durgical.com/en/clinical-skills/reading-a-full-blood-count/)
- [Liver tests: hepatocellular or cholestatic?](https://durgical.com/en/clinical-skills/liver-tests-hepatocellular-or-cholestatic/)

## Related conditions 10

- [Hyponatraemia](https://durgical.com/en/conditions/hyponatraemia/)
- [Hypernatraemia](https://durgical.com/en/conditions/hypernatraemia/)
- [Hyperkalaemia](https://durgical.com/en/conditions/hyperkalaemia/)
- [Hypokalaemia](https://durgical.com/en/conditions/hypokalaemia/)
- [Acute kidney injury](https://durgical.com/en/conditions/acute-kidney-injury/)
- [Chronic kidney disease](https://durgical.com/en/conditions/chronic-kidney-disease/)
- [Syndrome of inappropriate antidiuretic hormone secretion (SIADH)](https://durgical.com/en/conditions/syndrome-of-inappropriate-antidiuretic-hormone-secretion/)
- [Adrenal insufficiency](https://durgical.com/en/conditions/adrenal-insufficiency/)
- [High anion gap metabolic acidosis](https://durgical.com/en/conditions/high-anion-gap-metabolic-acidosis/)
- [Normal anion gap (hyperchloraemic) metabolic acidosis](https://durgical.com/en/conditions/normal-anion-gap-metabolic-acidosis/)

## In Pharmacology 6

- [Calcium gluconate](https://durgical.com/en/pharmacology/calcium-gluconate/)
- [Insulins](https://durgical.com/en/pharmacology/classes/insulins/)
- [Salbutamol](https://durgical.com/en/pharmacology/salbutamol/)
- [Potassium binders](https://durgical.com/en/pharmacology/classes/potassium-binders/)
- [Loop diuretics](https://durgical.com/en/pharmacology/classes/loop-diuretics/)
- [Tolvaptan](https://durgical.com/en/pharmacology/tolvaptan/)

[← All clinical skills](https://durgical.com/en/clinical-skills/)

Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: [Hyponatremia](https://www.ncbi.nlm.nih.gov/books/NBK470386/) (StatPearls (NCBI Bookshelf), 2026); [Hyperkalemia](https://www.ncbi.nlm.nih.gov/books/NBK470284/) (StatPearls (NCBI Bookshelf), 2025); [Creatinine Clearance](https://www.ncbi.nlm.nih.gov/books/NBK544228/) (StatPearls (NCBI Bookshelf), 2024); [Biochemistry, Anion Gap](https://www.ncbi.nlm.nih.gov/books/NBK539757/) (StatPearls (NCBI Bookshelf), 2023).

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Source: https://durgical.com/en/clinical-skills/electrolytes-urea-and-creatinine/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/clinical-skills/electrolytes-urea-and-creatinine/