4.5 Clinical Chemistry, Metabolic Panels & Reference Ranges
Key Takeaways
- A fasting plasma glucose of 70 to 99 mg/dL is normal, 100 to 125 mg/dL indicates prediabetes, and 126 mg/dL or higher on two occasions is diagnostic of diabetes.
- Hemoglobin A1C reflects average glycemia over roughly the preceding three months: below 5.7 percent is normal, 5.7 to 6.4 percent is prediabetes, and 6.5 percent or higher is diagnostic of diabetes.
- Kidney function is assessed with blood urea nitrogen (about 7 to 20 mg/dL) and creatinine (about 0.6 to 1.2 mg/dL) plus the calculated estimated glomerular filtration rate.
- Serum potassium of 3.5 to 5.0 mEq/L is a narrow, life-critical range, and hemolysis of the specimen releases intracellular potassium and falsely elevates the result.
- A critical (panic) value is a result so far outside the reference range that it threatens life; it is telephoned to the provider immediately and the read-back and notification are documented.
How Chemistry Specimens Behave
Most chemistry testing is performed on serum (from a red-top or gold/tiger-top serum separator tube, allowed to clot 30 minutes before centrifugation) or plasma (from a green-top heparin tube, which can be spun immediately). Three pre-analytical errors dominate the exam:
- Hemolysis ruptures red cells and releases their contents into the specimen. It falsely elevates potassium, lactate dehydrogenase, and phosphorus. A grossly hemolyzed potassium is the classic cause of a spurious hyperkalemia panic value.
- Delayed separation lets glucose be consumed by cells in the tube, falsely lowering glucose by roughly 5 to 7 percent per hour. A gray-top tube containing sodium fluoride blocks glycolysis and preserves glucose when separation will be delayed.
- Prolonged tourniquet time beyond one minute causes hemoconcentration and falsely elevates protein-bound analytes and potassium.
Glucose and Glycemic Control
| Test | Normal | Prediabetes | Diabetes |
|---|---|---|---|
| Fasting plasma glucose | 70–99 mg/dL | 100–125 mg/dL | ≥126 mg/dL |
| Hemoglobin A1C | <5.7% | 5.7–6.4% | ≥6.5% |
| 2-hour value, 75 g oral glucose tolerance test | <140 mg/dL | 140–199 mg/dL | ≥200 mg/dL |
| Random glucose with classic symptoms | — | — | ≥200 mg/dL |
Fasting means no caloric intake for at least 8 hours; water is permitted. Hemoglobin A1C measures the fraction of hemoglobin that is glycated and therefore reflects average glucose over the roughly 120-day red cell lifespan, weighted toward the most recent 8 to 12 weeks. It requires no fasting, which is why it is convenient for a walk-in visit, but it is unreliable in conditions that shorten red cell survival such as hemolytic anemia, recent transfusion, or advanced kidney disease.
The Metabolic Panels
A basic metabolic panel (BMP) contains eight analytes: sodium, potassium, chloride, carbon dioxide (bicarbonate), blood urea nitrogen, creatinine, glucose, and calcium. A comprehensive metabolic panel (CMP) adds six liver and protein markers: total protein, albumin, total bilirubin, alkaline phosphatase, alanine aminotransferase (ALT), and aspartate aminotransferase (AST).
| Analyte | Typical adult reference range |
|---|---|
| Sodium | 135–145 mEq/L |
| Potassium | 3.5–5.0 mEq/L |
| Chloride | 98–107 mEq/L |
| Carbon dioxide (bicarbonate) | 23–29 mEq/L |
| Blood urea nitrogen (BUN) | 7–20 mg/dL |
| Creatinine | 0.6–1.2 mg/dL |
| Calcium | 8.5–10.5 mg/dL |
Kidney function. BUN and creatinine both rise as filtration falls, but creatinine is the more specific marker because BUN also rises with dehydration, high protein intake, and gastrointestinal bleeding. The estimated glomerular filtration rate (eGFR) is calculated from creatinine, age, and sex; a value below 60 mL/min/1.73 m² sustained for three months defines chronic kidney disease.
Liver function. ALT is relatively liver-specific; AST is also present in cardiac and skeletal muscle. Both rise with hepatocellular injury. Alkaline phosphatase and bilirubin rise more prominently with biliary obstruction, and albumin falls with chronic liver disease because the liver synthesizes it.
A specimen for a basic metabolic panel is drawn with a difficult venipuncture requiring a two-minute tourniquet time, and the serum is visibly pink. The reported potassium is 6.1 mEq/L. What is the most appropriate interpretation?
Lipid Profile
| Component | Desirable adult value |
|---|---|
| Total cholesterol | <200 mg/dL |
| Low-density lipoprotein (LDL) | <100 mg/dL |
| High-density lipoprotein (HDL) | >40 mg/dL men, >50 mg/dL women (higher is protective) |
| Triglycerides | <150 mg/dL |
A useful memory aid is H for happy, L for lousy: HDL carries cholesterol away from the vessel wall to the liver, so a high value is favorable, while LDL deposits it in the arterial intima. Historically a 9- to 12-hour fast was required, driven mainly by the effect of a recent meal on triglycerides; current practice increasingly accepts nonfasting lipid panels for routine risk screening, so always follow the ordering provider's instruction rather than a memorized rule.
Thyroid Function
Thyroid-stimulating hormone (TSH) is the first-line screening test, with a typical range of about 0.4 to 4.0 mIU/L. The relationship is inverse, and this is the point the exam tests: in primary hypothyroidism the failing thyroid produces too little hormone, so the pituitary drives TSH up while free T4 falls. In hyperthyroidism, excess circulating hormone suppresses TSH down while free T4 rises.
| Condition | TSH | Free T4 | Typical presentation |
|---|---|---|---|
| Primary hypothyroidism | High | Low | Fatigue, weight gain, cold intolerance, constipation, bradycardia |
| Hyperthyroidism | Low | High | Weight loss, heat intolerance, tremor, tachycardia, anxiety |
Reference Ranges, Critical Values, and Escalation
A reference range is the interval containing about 95 percent of results from a healthy reference population. It is method-specific and laboratory-specific, so a result is always interpreted against the range printed on that report, and it varies with age, sex, pregnancy, and altitude.
A critical value, also called a panic value, is a result so far outside the reference range that it represents an immediate threat to life. Each laboratory publishes its own list; common examples include glucose below 40 or above 500 mg/dL, potassium below 2.5 or above 6.5 mEq/L, and hemoglobin below 7 g/dL.
The handling protocol is fixed and is examined frequently:
- The laboratory telephones the result to a licensed provider or a designated clinical staff member — it is never left on voicemail, sent only by fax, or routed to a queue for later review.
- The receiver performs a read-back: repeat the patient name, test, and value aloud and have the caller confirm.
- Document the date, time, the name of the caller, the name of the receiver, the value, the read-back, and the provider notification in the patient's record.
- Notify the provider immediately and document the response.
The medical assistant reports the number and the notification. Interpreting the result into a diagnosis or a treatment change is the provider's responsibility.
A patient's laboratory report shows TSH 12.8 mIU/L (reference 0.4–4.0) with a low free T4. Which interpretation is correct?
The reference laboratory telephones the office with a critical potassium value of 6.8 mEq/L. Which sequence reflects correct handling?