Understand chemistry analysers and the metabolic panel, calculate an anion gap and tell real results from artefacts.
Why it matters
Clinical chemistry is usually the busiest department in a hospital lab. Its analysers run thousands of tests a day on serum, plasma and urine: electrolytes, kidney and liver markers, glucose, lipids, proteins, enzymes, hormones and drugs. An MLS does not diagnose, but must recognize when a set of results does not make sense (interference, contamination, a mislabelled tube) and when it makes urgent sense (a critical value that needs a phone call now).
Scope. The numbers in this lesson are illustrative teaching values, not reference ranges to apply to anyone. Every laboratory establishes and validates its own reference intervals for its methods and population. Interpreting results for a patient is the care team's job; nothing here is medical advice.
How the analysers measure
- Ion-selective electrodes (ISE) measure sodium, potassium and chloride as a voltage across a selective membrane.
- Photometric (color) reactions measure many analytes, such as glucose and creatinine, by the absorbance of a reaction product (Beer-Lambert law).
- Enzymatic rate methods measure enzyme activity (ALT, AST, ALP) by how fast an absorbance changes.
- Immunoassays use antibodies to measure hormones, cardiac markers such as troponin, and drugs.
Each method has known interferences: hemolysis, lipemia, icterus, and some drugs. The analyser's HIL indices help the lab decide whether a result can be reported.
The basic and comprehensive metabolic panels
The basic metabolic panel (BMP) is eight tests:
| Test | What it reflects | Illustrative adult value |
|---|---|---|
| Sodium (Na) | Water balance | about 135 to 145 mmol/L |
| Potassium (K) | Heart and muscle function | about 3.5 to 5.0 mmol/L |
| Chloride (Cl) | Acid-base and fluid balance | about 98 to 107 mmol/L |
| Bicarbonate (CO2) | Acid-base status | about 22 to 29 mmol/L |
| Urea nitrogen (BUN) | Protein breakdown, kidney clearance | about 7 to 20 mg/dL |
| Creatinine | Kidney filtration | about 0.6 to 1.2 mg/dL |
| Glucose | Blood sugar | about 70 to 99 mg/dL fasting |
| Calcium | Bone, nerve, muscle | about 8.5 to 10.5 mg/dL |
The comprehensive metabolic panel (CMP) adds total protein, albumin, bilirubin, ALP, ALT and AST, which reflect liver function and nutrition. (All values above are illustrative only.)
Reading patterns, not single numbers
Two useful calculations:
- Anion gap = Na - (Cl + HCO3). With Na 140, Cl 104 and HCO3 24, the gap is 140 - 128 = 12 mmol/L, in the usual teaching range of about 8 to 16. A high gap points the care team toward certain kinds of metabolic acidosis.
- BUN-to-creatinine ratio. A ratio well above about 20 is classically associated with reduced blood flow to the kidneys (a "prerenal" pattern), while a ratio near 10 to 15 with both raised fits a problem in the kidney itself.
The lab's interest in patterns is also about plausibility:
Worked example: is this result real?
Three fictional potassium results arrive in an hour:
- K 6.8 mmol/L, hemolysis index high, other results normal. Hemolysis releases potassium from red cells. The MLS reports per policy (usually "hemolyzed, recollect") instead of calling it as a critical value.
- K 6.8 mmol/L, no hemolysis, creatinine 4.5 mg/dL, previous K 6.1. This fits a real, worsening problem consistent with kidney failure. It is a critical value: verify per policy and call it now (lesson
clin-m3-l1-critical). - K 9.1 mmol/L with calcium 2.0 mg/dL in a patient whose results yesterday were normal. The pattern (very high K, very low Ca) suggests EDTA contamination. Request a redraw.
The tool behind examples 2 and 3 is the delta check: the LIS compares a result with the patient's previous one and flags changes too large to be biologically likely. A delta-check failure prompts the MLS to look for a mislabel, contamination or instrument problem before releasing the result.
Common mistakes
- Treating a single out-of-range value as a diagnosis instead of a prompt to check the specimen and the pattern.
- Releasing a hemolyzed potassium without a comment.
- Ignoring a delta-check flag because the value is "only slightly high."
- Quoting reference ranges from the internet; the lab's own validated intervals are the only ones that apply to its results.
Quick recap
- Chemistry analysers use ISE, photometry, enzyme rates and immunoassays, each with known interferences.
- BMP: Na, K, Cl, CO2, BUN, creatinine, glucose, calcium; CMP adds liver tests and proteins.
- Anion gap = Na - (Cl + HCO3); the BUN/creatinine ratio hints at prerenal vs kidney patterns.
- Plausibility checks (HIL indices, delta checks, known contamination patterns) protect the patient.
Try it
Open the Do tab for the quiz: calculate an anion gap and decide whether three results are real.