investigations done in general medicine
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Interpretation of Blood Tests in General Medicine
Here is a systematic guide to interpreting the main blood investigations used in clinical practice.
1. Full Blood Count (FBC / CBC)
The FBC is fully automated and provides three main groups of results.
Red Cell Parameters
| Parameter | What it Measures | Clinical Implication |
|---|---|---|
| Hb (Haemoglobin) | Total Hb in blood | Low = anaemia |
| RCC (Red Cell Count) | Number of RBCs | Can be high even with low Hb (e.g. thalassaemia) |
| MCV (Mean Corpuscular Volume) | Size of RBC | Low = microcytic; High = macrocytic |
| MCH (Mean Corpuscular Haemoglobin) | Amount of Hb per cell | Low in iron deficiency |
| MCHC | Hb concentration inside cells | Low in iron deficiency, high in hereditary spherocytosis |
| Reticulocyte count | Young RBCs from bone marrow | High = marrow actively producing RBCs (haemolysis or blood loss recovery) |
Interpreting Anaemia by MCV:
- Microcytic anaemia (low MCV): Iron deficiency, thalassaemia, anaemia of chronic disease, sideroblastic anaemia
- Normocytic anaemia (normal MCV): Acute blood loss, anaemia of chronic disease, renal failure, haemolysis
- Macrocytic anaemia (high MCV): B12/folate deficiency (megaloblastic), alcohol, hypothyroidism, liver disease, drugs (methotrexate, hydroxyurea)
White Cell Parameters (5-Part Differential)
| Cell | Normal Range | High (causes) | Low (causes) |
|---|---|---|---|
| Neutrophils | 2-7 x10⁹/L | Bacterial infection, steroids, stress, malignancy | Viral infection, drug toxicity, marrow failure |
| Lymphocytes | 1.5-4 x10⁹/L | Viral infection (EBV, CMV), CLL, TB | HIV, steroids, SLE |
| Eosinophils | 0.04-0.4 x10⁹/L | Parasites, allergy, asthma, Addison's | Steroids |
| Monocytes | 0.2-1.0 x10⁹/L | TB, malaria, inflammation | - |
| Basophils | 0-0.1 x10⁹/L | CML, allergic reactions | - |
Platelets
- Thrombocytopenia (<150 x10⁹/L): ITP, heparin-induced, DIC, hypersplenism, marrow failure
- Thrombocytosis (>400 x10⁹/L): Reactive (infection, iron deficiency, post-surgery), essential thrombocythaemia
Pancytopenia (low RBCs + WBCs + platelets) occurs from:
- Bone marrow failure: aplastic anaemia, acute leukaemia, myelodysplasia, myeloma
- Infiltration: lymphoma, TB, solid tumours
- Megaloblastic anaemia
- Increased peripheral destruction: hypersplenism, haemophagocytic syndrome
(Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol 1)
2. Liver Function Tests (LFTs)
The liver has three functional systems that are tested by different markers.
Hepatocellular Damage Markers
| Test | Significance |
|---|---|
| ALT (Alanine Aminotransferase) | Most specific for hepatocyte damage; very high in viral/toxic hepatitis |
| AST (Aspartate Aminotransferase) | Also elevated in hepatitis; rises in myocardial infarction too |
| LDH | Rises in hepatocyte injury and necrosis |
ALT:AST ratio >2 suggests alcoholic liver disease; if AST dominates, think MI or muscle disease.
Cholestatic / Obstructive Markers
| Test | Significance |
|---|---|
| ALP (Alkaline Phosphatase) | Rises in biliary obstruction, bone disease, pregnancy |
| GGT (Gamma-Glutamyl Transferase) | Confirms ALP is of hepatic origin; very sensitive to alcohol |
| Bilirubin (total + direct) | High direct = conjugated/obstructive; High indirect = haemolysis or Gilbert's |
Synthetic Function Markers
| Test | Significance |
|---|---|
| Albumin | Reflects chronic liver synthetic function; falls only when >80% liver is destroyed |
| Total Protein | Follows albumin trends |
| PT / INR | Clotting factors made by liver; prolonged in hepatic failure |
| Ammonia | Rises when >80% liver is destroyed; causes hepatic encephalopathy |
Six Patterns of LFT Interpretation
- Acute hepatitis / necrosis - Markedly elevated ALT + AST, elevated direct bilirubin, normal ALP
- Cholestasis / biliary obstruction - Elevated ALP, GGT, bilirubin (mainly direct), mildly elevated transaminases
- Alcoholic liver disease - GGT disproportionately high, AST:ALT >2
- Cirrhosis - Low albumin, prolonged PT, variable bilirubin
- Hepatic infiltration/metastases - Elevated ALP, mildly raised transaminases
- Haemolysis - Elevated indirect (unconjugated) bilirubin, normal ALP, elevated LDH
(Henry's Clinical Diagnosis and Management by Laboratory Methods)
3. Renal Function Tests (RFTs)
| Test | Normal Range | Interpretation |
|---|---|---|
| Urea (BUN) | 2.5-7.1 mmol/L | Raised in prerenal (dehydration, GI bleed), renal failure; low in liver disease |
| Creatinine | 60-110 µmol/L (male) | Rises when GFR drops significantly; affected by muscle mass |
| eGFR | >60 mL/min/1.73m² | Best routine measure of kidney function; used to stage CKD |
| Uric acid | 200-430 µmol/L | Raised in gout, myeloproliferative disease, renal failure |
Urea:Creatinine Ratio:
-
100 (or >20 in US units) = Prerenal cause (dehydration, GI bleed, heart failure)
- Normal ratio = Intrinsic renal disease or postrenal obstruction
4. Electrolytes
| Electrolyte | Normal | High (causes) | Low (causes) |
|---|---|---|---|
| Sodium (Na⁺) | 135-145 mmol/L | Dehydration, DI, excess NaCl | SIADH, hypothyroidism, heart failure, diarrhoea |
| Potassium (K⁺) | 3.5-5.0 mmol/L | Renal failure, ACEi/ARBs, Addison's, acidosis | Diarrhoea, diuretics, Cushing's, alkalosis |
| Chloride (Cl⁻) | 95-107 mmol/L | Dehydration, renal failure | Vomiting, metabolic alkalosis |
| Bicarbonate (HCO₃⁻) | 22-29 mmol/L | Metabolic alkalosis | Metabolic acidosis, diarrhoea |
| Calcium (Ca²⁺) | 2.2-2.6 mmol/L | Hyperparathyroidism, malignancy, sarcoid, Vit D toxicity | Hypoparathyroidism, Vit D deficiency, malabsorption |
| Phosphate | 0.8-1.5 mmol/L | Renal failure, hypoparathyroidism | Malnutrition, hyperparathyroidism, refeeding syndrome |
| Magnesium | 0.7-1.0 mmol/L | Renal failure | Diarrhoea, alcohol, diuretics, PPIs |
Anion Gap = Na - (Cl + HCO₃), normal 8-12 mmol/L
- Raised AG acidosis (MUDPILES): Methanol, Uraemia, DKA, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates
- Normal AG (hyperchloraemic) acidosis: Diarrhoea, renal tubular acidosis, acetazolamide
5. Blood Glucose
| Test | Interpretation |
|---|---|
| Fasting glucose <6.1 mmol/L | Normal |
| 6.1-6.9 mmol/L | Impaired fasting glucose (pre-diabetes) |
| ≥7.0 mmol/L (on 2 occasions) | Diabetes mellitus |
| Random glucose ≥11.1 mmol/L | Diabetes mellitus (with symptoms) |
| HbA1c ≥48 mmol/mol (6.5%) | Diabetes mellitus |
| HbA1c 42-47 mmol/mol | Pre-diabetes |
6. Thyroid Function Tests (TFTs)
| Pattern | TSH | Free T4 | Free T3 | Diagnosis |
|---|---|---|---|---|
| Primary hypothyroidism | High | Low | Low | Hashimoto's, iodine deficiency |
| Subclinical hypothyroidism | High | Normal | Normal | Early Hashimoto's |
| Primary hyperthyroidism | Low/undetectable | High | High | Graves', toxic nodule |
| Subclinical hyperthyroidism | Low | Normal | Normal | Early Graves', excess thyroxine |
| Secondary hypothyroidism | Low/normal | Low | Low | Pituitary failure |
7. Inflammatory Markers
| Test | Uses | Limitations |
|---|---|---|
| CRP (C-Reactive Protein) | Rises within 6-12 hours of inflammation/infection; falls quickly on recovery | Non-specific |
| ESR (Erythrocyte Sedimentation Rate) | Rises more slowly; useful in PMR, myeloma, TB | Affected by age, sex, anaemia, pregnancy |
| WBC | Neutrophilia = bacterial; Lymphocytosis = viral | - |
| Procalcitonin | More specific for bacterial sepsis | Expensive, not universally available |
| Ferritin | Acute phase reactant; also measures iron stores | Very high in haemophagocytic syndrome, Still's disease |
8. Coagulation Screen
| Test | Normal | Prolonged in |
|---|---|---|
| PT / INR | INR 0.9-1.2 | Warfarin, liver disease, Vit K deficiency, DIC |
| APTT | 26-37 seconds | Heparin, haemophilia A/B, lupus anticoagulant, DIC |
| Fibrinogen | 2-4 g/L | Decreased in DIC, liver failure (consumed) |
| D-dimer | <0.5 mg/L | Raised in VTE, DIC, malignancy, post-surgery (sensitive, not specific) |
Key Clinical Principles
- Always interpret results in clinical context - a "normal" result may still be abnormal for a particular patient (e.g. a creatinine of 90 in a young muscular male vs. a frail elderly woman).
- Trends matter more than single values - a creatinine rising from 80 to 120 is more significant than a stable creatinine of 130.
- Check specimen quality - clotted, haemolysed, or delayed samples give spurious results, especially for potassium and LDH.
- Correct for confounders - albumin-corrected calcium, MCHC-adjusted MCV, and medication effects on results.
Create a quick reference PDF with interpretation flowcharts for each test
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PDF generated successfully (42 KB). Here it is:
Blood Test Interpretation QuickRef
PDF Document · PDF
What's inside (10 pages):
| Page | Content |
|---|---|
| 1 | Cover page with colour branding and test index |
| 2 | FBC - Anaemia by MCV flowchart, dual micro/macrocytic branches, WBC differential table, platelet & pancytopenia tables |
| 3 | LFTs - Hepatocellular vs cholestatic dual flowchart, bilirubin conjugation flowchart, LFT pattern summary table |
| 4 | RFTs - Urea:Cr ratio flowchart, AKI classification table, CKD G1-G5 staging table |
| 5 | Electrolytes - Na hypo/hypernatraemia dual flowchart, K hypo/hyperkalaemia dual flowchart with ECG clues, anion gap MUDPILES flowchart, calcium causes table |
| 6 | Glucose & HbA1c - Diabetes diagnosis flowchart with OGTT thresholds, HbA1c monitoring table, hypoglycaemia causes table |
| 7 | TFTs - TSH-first algorithm, high/low TSH dual flowchart, full pattern table including sick euthyroid |
| 8 | Inflammatory Markers - CRP/ESR interpretation flowchart, marker comparison table (kinetics, uses, limitations) |
| 9 | Coagulation - PT/APTT pathway flowchart, coag test table, ISTH DIC scoring table |
| 10 | Normal Ranges Summary - All 44 tests in two-column quick-lookup format |
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