SLLE Laboratory Complete Review Guide 2026–2027

4. Henderson-Hasselbalch: pH = 6.1 + log [HCO3 ÷ (0.03 × pCO2)]. Normal arterial values: pH 7.35 to 7.45, pCO2 35 to 45 mmHg, HCO3 22 to 26 mmol/L.
5. Metabolic acidosis: low HCO3, compensated by hyperventilation that lowers pCO2. Respiratory acidosis: high pCO2, compensated by the kidneys raising HCO3.
6. Anion gap = Na − (Cl + HCO3), normally about 8 to 12 mmol/L when K is not included. It rises in lactic acidosis, ketoacidosis, renal failure and poisoning with methanol, ethylene glycol or salicylate.
7. About half of serum calcium is ionized (the active form) and the rest is bound to albumin or small complexes. Acidosis raises ionized calcium. Phosphorus varies inversely with calcium through parathyroid hormone (PTH).
8. Creatinine comes from muscle creatine metabolism and is used to estimate the glomerular filtration rate (GFR). It is measured by the Jaffe (alkaline picrate) or enzymatic method. The BUN to creatinine ratio rises above the usual (about 10 to 20) in prerenal causes.
9. Iron patterns: in iron deficiency, iron and ferritin fall and TIBC rises; in chronic inflammation, iron and TIBC fall and ferritin is normal or high; in iron overload (hemochromatosis), iron and transferrin saturation (iron ÷ TIBC × 100) rise.
10. Cardiac troponin (cTnI and cTnT) is the most specific marker of myocardial infarction: it rises within hours and stays high for several days. CK-MB falls faster (within 2 to 3 days), and BNP and NT-proBNP are used to assess heart failure.
11. Bilirubin: unconjugated (indirect) rises in hemolysis; conjugated (direct) rises in biliary obstruction and appears in urine with reduced urobilinogen. The specimen is protected from light because bilirubin breaks down in it.
12. Enzymes: ALT and AST for hepatocellular injury (ALT is more liver-specific); ALP and GGT for cholestasis (GGT confirms a liver origin of ALP); lipase is more specific than amylase for pancreatitis; CK for muscle; LD rises in myocardial infarction and hemolysis.
13. Calculated LDL (Friedewald) = total cholesterol − HDL − TG ÷ 5 (mg/dL units), and it is not valid when TG exceeds about 400 mg/dL. Measuring TG needs fasting of about 9 to 12 hours, and chylomicrons form a creamy layer when serum stands in the refrigerator.
14. Thyroid: high TSH with low T4 indicates primary hypothyroidism, and low TSH with high T4 and T3 indicates primary hyperthyroidism. Total T4 rises when TBG rises, as in pregnancy and estrogen use.
15. Tumor markers: AFP for hepatocellular carcinoma and germ cell tumors, CEA for colorectal cancer, PSA for prostate, hCG for germ cell and trophoblastic tumors. They are used mainly for follow-up and response to treatment.
16. Therapeutic drug monitoring (TDM): the peak is drawn a set time after the dose and the trough just before the next dose. A drug reaches steady state after about 5 half-lives.
17. Urine drug testing starts with an immunoassay screen, and a positive result is confirmed by gas chromatography with mass spectrometry (GC-MS). Carbon monoxide is measured as carboxyhemoglobin by CO-oximetry, and a pulse oximeter may falsely read normal.
18. PCR steps: heat separation of the DNA strands (denaturation), primer binding (annealing), then extension by Taq polymerase. PCR detects known mutations, SSCP detects unknown mutations, and Southern blot detects copy number changes.

Common mistakes in this chapter

• ✕ Mixing up metabolic acidosis (low HCO3) and respiratory acidosis (high pCO2): first find the variable that moved in the same direction as the pH.
• ✕ Assuming compensation brings the pH fully back to normal: in a partly compensated disorder the pH stays outside the reference range.
• ✕ Accepting a high K as real: hemolysis, delayed serum separation and EDTA contamination all raise K falsely.
• ✕ Confusing total and ionized calcium: total calcium depends on albumin, while ionized calcium is the physiologically active form and depends on pH.
• ✕ Treating HbA1c as current glucose: it reflects about 2 to 3 months and can mislead in hemolytic anemia or with hemoglobin variants.
• ✕ Treating ALT and AST as equally liver-specific: ALT is more liver-specific, and AST also rises with muscle, heart and hemolysis.
• ✕ Applying the Friedewald equation when triglycerides exceed about 400 mg/dL, which makes the calculated LDL unreliable.
• ✕ Confusing the screening test (immunoassay) with the confirmatory test (GC-MS): the first is presumptive, the second confirms.