Clinical Biochemistry and Diagnostics: Blood and Urine Analysis, Liver and Kidney Function and Diabetes Markers
What this domain tests
Clinical biochemistry questions test blood and urine analysis, liver and kidney function tests, and diabetes markers, with emphasis on choosing and interpreting the right test.
Core rules
- Plasma glucose is preferred over serum for glucose because glycolysis continues in serum unless fluoride is added. Fasting plasma glucose >=126 mg/dL on two occasions, 2-hour OGTT >=200 mg/dL, or HbA1c >=6.5% supports diabetes. Random glucose >=200 mg/dL with symptoms is diagnostic.
- HbA1c reflects average glycemia over about 2-3 months; it is not a measure of acute control.
- Liver tests: ALT and AST reflect hepatocellular injury; ALP and GGT reflect cholestasis; bilirubin measures conjugation and excretion; albumin and prothrombin time reflect synthetic function. ALT > AST is typical of viral hepatitis; AST/ALT >2 suggests alcohol-related injury.
- Kidney tests: serum creatinine and urea, eGFR, and urinalysis. Creatinine depends on muscle mass; urea depends on hydration and protein intake. Proteinuria, hematuria, and casts are urine abnormalities.
- Urine dipstick can detect glucose, ketones, protein, blood, and nitrite.
Worked example
A fasting plasma glucose is 130 mg/dL and HbA1c is 7.0%. Both exceed diagnostic cutoffs (126 mg/dL and 6.5%), so the results support diabetes. A single random urine glucose is not diagnostic because renal threshold and hydration affect it. A serum creatinine of 1.0 mg/dL may be normal in a muscular adult but abnormal in a small elderly patient, so eGFR is more useful than creatinine alone.
Traps
HbA1c is not for diagnosing acute complications. Serum is not always better than plasma for glucose. ALT is not a kidney marker. Urea is not a direct measure of glomerular filtration. Bilirubin elevation does not by itself distinguish prehepatic, hepatic, and posthepatic causes.