10.1 Clinical Chemistry Terminology and Test Families
Key Takeaways
- Serum is the liquid remaining after blood has clotted; plasma is the liquid portion of anticoagulated blood — they are not interchangeable chemistry specimens.
- Hepatic (liver) function tests include AST, ALT, ALP, and bilirubin; kidney (renal) function tests include BUN and creatinine.
- Carbohydrate (glucose) metabolism tests include glucose and hemoglobin A1c; routine electrolytes are sodium, potassium, chloride, and CO2 (bicarbonate).
- After centrifugation, the liquid on top is the supernatant; pouring or pipetting it off the cells is to decant.
- Endocrinology tests measure hormones (TSH is the high-volume example); CMLA recognizes the order and preanalytic needs and does not independently run high-complexity chemistry analyzers.
CMLA Work Area III (Examination / Analytical Considerations) is 83 items, or 41.5% of the 200-question exam. Chemistry is competency 5. AMT III.5.A.1 requires you to know terminology related to clinical chemistry: hepatic (liver) function tests, carbohydrate (glucose) metabolism tests, kidney (renal) function tests, general chemistry tests, supernatant, endocrinology, decant, electrolytes, serum, and plasma.
A Certified Medical Laboratory Assistant (CMLA) performs waived and point-of-care (POC) chemistry and maintains those devices (Sections 10.2–10.3). You also collect and process specimens for the main-laboratory chemistry bench. You do not independently operate high-complexity automated chemistry analyzers. The words in this section are how you recognize an order, choose the tube the standard operating procedure (SOP) names, and describe what you pour after centrifugation.
Quick Answer: Serum is the liquid after blood clots; plasma is the liquid of anticoagulated blood. After spinning, the liquid on top is the supernatant; pouring it off is to decant. Hepatic tests (AST, ALT, ALP, bilirubin) assess the liver. Renal tests (BUN, creatinine) assess the kidney. Carbohydrate tests include glucose and A1c. Electrolytes are Na, K, Cl, and CO2. Endocrinology measures hormones such as TSH. Match the family to the requisition and the tube.
Serum versus plasma for chemistry
Whole blood is blood as drawn, with cells still suspended. Plasma is the liquid portion of anticoagulated blood — the specimen was mixed with an additive so a clot never formed. Serum is the liquid left after blood has clotted and the clot has retracted. Chapter 6 defined these three products. Chemistry is where the distinction decides the tube and whether a result is valid.
| Product | How you obtain it | Common chemistry containers | Classic CMLA trap |
|---|---|---|---|
| Serum | Allow a full clot, then centrifuge | Red top (clot, no additive) or gold/tiger serum separator tube (SST) | Calling SST gel "plasma" |
| Plasma | Mix the anticoagulant immediately, then centrifuge | Green lithium heparin (often used for rapid chemistry); some assays specify sodium heparin | Using EDTA (lavender) for potassium |
| Whole blood | Do not clot; do not separate | Waived glucose meters and other waived cartridges only if the instructions for use (IFU) name whole blood | Pouring serum or plasma onto a whole-blood strip |
Classic chemistry panels were built on serum. Many hospital analyzers now accept lithium heparin plasma so results can leave the bench without a 30-minute clot wait. That is an SOP and instrument-claim decision, not a CMLA improvisation. If the order is a basic metabolic panel (BMP) or comprehensive metabolic panel (CMP) and the SOP says green heparin, collect green heparin. If the SOP says SST, collect SST. You do not pour cells from one tube into another "to make serum," and you do not use leftover lavender ethylenediaminetetraacetic acid (EDTA) blood as a chemistry specimen.
Potassium (K+) is the reason EDTA is a chemistry disaster. EDTA in lavender-top tubes is a potassium salt. Residual EDTA plasma mixed into a chemistry aliquot falsely raises potassium and chelates calcium and magnesium. Hemolysis (broken red cells) also dumps intracellular potassium into the liquid and falsely raises K+. Recognize the order, the tube, and the hemolysis reject. Do not "rescue" a lavender tube by pouring it into an SST.
Supernatant and decant
After centrifugation, two layers form. The supernatant is the clear or straw-colored liquid on top — serum or plasma. The cells (and, in serum, the clot) sit on the bottom. In an SST, a gel barrier should sit between them if the tube was spun as directed.
To decant is to pour the supernatant off the cells into another labeled container (an aliquot tube). Pipetting the same liquid is the cleaner version of the same idea. You decant or pipette when the SOP requires a pour-off for send-out chemistry, when the analyzer cannot accept a gel tube, or when cells must not remain in contact with the liquid during delay. You do not remix the tube and decant a muddy mixture. You do not decant the clot. You do not leave plasma sitting on cells beyond the SOP time limit (Chapter 7). If the supernatant is hemolyzed (pink or red), icteric (deep yellow), or lipemic (milky), document the appearance; many chemistry assays are invalid in those matrices.
Scenario. A centrifuged green-top plasma tube sits on the rack. The liquid on top is the supernatant. You pipette that plasma into a labeled aliquot for a send-out hepatic panel. You have transferred (decanted) the supernatant. If you invert the original tube first "to mix," you have resuspended cells and ruined the aliquot.
Hepatic (liver) function tests
Hepatic function tests (often called liver function tests, LFTs) are chemistry assays that reflect hepatocyte injury, biliary obstruction, or synthetic function. A CMLA does not diagnose hepatitis from a number. You recognize that an hepatic panel or LFT order belongs in the liver family, collect the named tube, protect the specimen, and send it.
| Test | Abbreviation | What clinicians watch | Preanalytic note for the assistant |
|---|---|---|---|
| Aspartate aminotransferase | AST | Hepatocyte injury (also muscle) | Hemolysis can elevate AST |
| Alanine aminotransferase | ALT | More liver-specific injury | Serum or heparin plasma as the SOP names |
| Alkaline phosphatase | ALP | Liver or bone; biliary tree | Age and pregnancy change expected values; do not invent a "normal" |
| Bilirubin (total ± direct) | — | Conjugation and excretion | Protect some neonatal samples from light per SOP |
| Albumin / total protein | — | Synthetic function (often on a CMP) |
Gamma-glutamyl transferase (GGT) may appear on some hepatic panels. Know it as another liver-associated enzyme, not as a waived meter test.
Kidney (renal) function tests
Kidney (renal) function tests assess how well the kidneys clear waste.
- Blood urea nitrogen (BUN) measures urea nitrogen, a protein-breakdown waste.
- Creatinine is produced from muscle creatine and is the usual filtration marker.
- Many reports include a calculated estimated glomerular filtration rate (eGFR) from creatinine, age, and sex. You do not compute eGFR by hand on CMLA; you recognize it as a renal result, not a separate tube.
A BMP almost always includes BUN, creatinine, and glucose with the electrolytes. Isolated "renal panel" orders still need the same preanalytic care: correct tube, no EDTA contamination, no hemolysis, prompt separation from cells.
Carbohydrate metabolism, electrolytes, and general chemistry
Carbohydrate (glucose) metabolism tests include glucose (fasting, random, or timed) and hemoglobin A1c (A1c, glycated hemoglobin). A1c reflects average glucose over roughly the prior two to three months. Gray-top sodium fluoride (often paired with oxalate) inhibits glycolysis when laboratory glucose will be delayed — a preservative from Chapter 6, not a waived-meter strip. Some A1c devices are waived (Section 10.3); many laboratory A1c assays are not. Do not call A1c a renal test or a liver test, and do not treat a fingerstick glucose as an A1c.
Electrolytes in routine chemistry are sodium (Na+), potassium (K+), chloride (Cl−), and carbon dioxide (CO2) / bicarbonate. They live on the BMP. Critical sodium and potassium values are report-immediately results (Chapter 17). Here you need the family name and the hemolysis/EDTA potassium traps.
General chemistry tests is AMT's umbrella for common panels and related analytes:
- BMP: typically Na, K, Cl, CO2, BUN, creatinine, glucose, and often calcium.
- CMP: the BMP group plus hepatic tests (AST, ALT, ALP, bilirubin) and protein/albumin.
- Other general chemistry you must recognize on a requisition: magnesium, phosphorus, a lipid panel (cholesterol, HDL, LDL, triglycerides), and sometimes enzymes such as creatine kinase. Lipids may be fasting-sensitive depending on the IFU and SOP.
Endocrinology
Endocrinology is the laboratory measurement of hormones. The CMLA outline does not require you to operate a thyroid immunoassay analyzer. It requires you to know the family. The highest-volume example is thyroid-stimulating hormone (TSH). Related thyroid orders (free thyroxine) and other hormones (cortisol, reproductive hormones) are usually processed as serum or the SOP-named plasma and sent to the immunoassay bench. Collection timing (morning cortisol, fasting) is a preanalytic instruction, not a waived chemistry strip. Human chorionic gonadotropin (hCG) may be a waived urine cassette (Chapter 12) or a quantitative blood test — do not mix those methods.
Exam traps
- Serum and plasma are not synonyms; EDTA plasma is not a chemistry potassium specimen.
- Supernatant is the top liquid, not the cell pellet; decant is the pour-off, not remixing.
- AST/ALT/ALP/bilirubin are hepatic; BUN/creatinine are renal; glucose/A1c are carbohydrate; Na/K/Cl/CO2 are electrolytes; TSH is endocrinology.
- A CMP is not "just electrolytes."
- Knowing the family is not a license to independently operate a high-complexity chemistry analyzer.
When CMLA shows a chemistry word, put it in a family, name the usual specimen, and name the look-alike it is not.
Which statement correctly distinguishes serum from plasma for clinical chemistry?
After a chemistry tube is centrifuged, the CMLA must transfer the liquid without transferring cells. Which description uses the official terms correctly?
A provider orders a comprehensive metabolic panel (CMP). Which grouping is correct?