Clinical Biochemistry & Interpreting Blood Tests
U&Es, LFTs, eGFR and how to interpret laboratory results in clinical practice
Blood tests are one of the most important tools in clinical practice. Being able to interpret results correctly — and understanding how drugs affect them — is a core pharmacist competency. This article explains the key tests you need to know and how to apply them in practice.
Why Do Blood Tests?
Blood tests serve four main purposes in clinical practice. Diagnosis: to confirm or exclude a condition (e.g. raised TSH confirms hypothyroidism). Prognosis: to predict how a condition will progress (e.g. eGFR trajectory in CKD). Screening: to detect disease before symptoms appear (e.g. HbA1c for pre-diabetes). Monitoring: to track treatment response and detect drug-related side effects (e.g. LFTs on methotrexate, potassium on ACE inhibitors). Pharmacists need to understand all four uses to support safe prescribing and medicines monitoring.
Understanding Reference Ranges
Reference ranges are derived from the middle 95% of results in a healthy population — which means 5% of entirely healthy people will have results outside the normal range. A result outside the reference range does not automatically indicate disease. Biological variation is significant: age, sex, pregnancy, diet, posture, exercise, and time of day all affect results. Creatinine is lower in elderly patients (reduced muscle mass), albumin falls in pregnancy, ALP rises in bone growth and liver disease. Context is everything — always interpret results alongside the clinical picture and the patient's drug list.
Common Biochemistry Tests — U&Es
Urea and electrolytes (U&Es) are among the most frequently requested tests. Sodium (135–145 mmol/L) reflects water balance — low sodium often indicates SIADH (caused by SSRIs, carbamazepine, and many other drugs). Potassium (3.5–5.0 mmol/L) is critical for cardiac rhythm — ACE inhibitors and potassium-sparing diuretics raise it; loop and thiazide diuretics lower it. Creatinine and eGFR reflect kidney function; the eGFR is used for CKD staging while Cockcroft-Gault is used for drug dosing. Systematic evaluation of all U&E results together gives more information than looking at individual values in isolation.
Liver Function Tests (LFTs)
LFTs measure several markers of liver health. ALT and AST are enzymes that rise when liver cells are damaged (hepatocellular pattern). ALP and GGT rise when bile flow is blocked (cholestatic pattern). An AST:ALT ratio above 2 suggests alcoholic liver disease. An isolated GGT rise can indicate alcohol use or enzyme-inducing drugs (carbamazepine, phenytoin). Albumin reflects the liver's ability to make proteins — low albumin indicates chronic liver disease, malnutrition, or nephrotic syndrome. Careful evaluation of the pattern of abnormality determines whether the picture is hepatocellular or cholestatic — this guides the next steps in investigation and management.