Interpreting lab tests

Blood count and red cell indices

• Pregnancy: plasma volume rises about 50% but red cell mass only about 25%, so Hb falls (for example 12.5 to 11 g/dL) with normal MCV and ferritin: physiological haemodilution.
• Beta-thalassaemia trait: very low MCV with near-normal Hb, raised HbA2 (over 3.5%), Mentzer index (MCV/RBC) under 13.

• ESR rises with myeloma, giant cell arteritis and polymyalgia (often over 100 mm/h), TB and infection; it is low in sickle cell disease and polycythaemia.

Iron studies and haemolysis

• Ferritin is an acute phase protein, so it also rises in inflammation; transferrin saturation separates iron overload from inflammation.
• Haemolysis: raised unconjugated bilirubin, LDH and reticulocytes; low haptoglobin.
• PNH: complement destroys red cells lacking the GPI-anchored protectors CD55 and CD59. Dark morning urine, pancytopenia and hepatic vein thrombosis (Budd–Chiari); confirm by flow cytometry for CD55/CD59 (or FLAER).
• HbA1c reflects glucose exposure over the red cell lifespan. Where lifespan is shortened (sickle cell, haemolysis, recent bleeding), HbA1c is falsely low; use glucose or fructosamine.
• ABO and Rh typing: red cells agglutinate with the antisera matching their antigens; plasma holds antibodies to the antigens the person lacks. Agglutination with anti-A and anti-D, with anti-B in the plasma, is group A Rh-positive.

Kidney, electrolytes and acid–base

• High urea:creatinine ratio: pre-renal AKI, upper GI bleed (digested blood protein), catabolism. Low urea: liver failure, malnutrition, pregnancy.
• Pre-renal versus ATN: urine sodium under 20 versus over 40 mmol/L; FENa under 1% versus over 2%; bland urine versus muddy brown granular casts.
• Uraemic pericarditis (chest pain, friction rub, very high urea) is an indication for urgent dialysis; minimise heparin because the effusion may be bloody. Other indications: refractory hyperkalaemia, acidosis, fluid overload, encephalopathy.
• Low calcium, high phosphate, high PTH: chronic kidney disease (secondary hyperparathyroidism) or pseudohypoparathyroidism (PTH resistance). Vitamin D status is measured as 25-hydroxyvitamin D, the storage form.
• Primary hyperaldosteronism (Conn adenoma): hypertension with low potassium, metabolic alkalosis and suppressed renin (high aldosterone:renin ratio). High renin with hypertension suggests renal artery stenosis.
• Thiazide hyponatraemia with hypovolaemia (Na 119, low osmolality): stop the thiazide, give isotonic saline, correct slowly.
• Hyperkalaemia (for example 6.7 mmol/L in a patient on ACE inhibitors or spironolactone) gives tall T waves then wide QRS and arrest.
• Diarrhoea loses bicarbonate and potassium: normal anion gap (hyperchloraemic) metabolic acidosis with hypokalaemia (for example Cl 112, K 2.5, anion gap 10).

• Cystinuria: autosomal recessive defect in proximal tubular reabsorption of the dibasic amino acids cystine, ornithine, lysine and arginine (COLA). Only cystine is insoluble, forming flat hexagonal crystals and stones; cyanide–nitroprusside test is positive.
• ABG normals: pH 7.35–7.45; PaCO2 4.7–6.0 kPa (35–45 mmHg); PaO2 over 10.6 kPa on air; HCO3− 22–26. Type 1 respiratory failure: PaO2 under 8 kPa with normal or low PaCO2; type 2: PaCO2 over 6.5 kPa. Expected PaCO2 in metabolic acidosis (Winter formula) = 1.5 × HCO3− + 8 ± 2 mmHg.

Liver tests and hepatitis serology

• Jaundice is visible above about 35–50 µmol/L (2–3 mg/dL); normal under 21 µmol/L (1.2 mg/dL).
• Anti-HCV positive with normal LFTs: antibody shows exposure, not current infection; next test HCV RNA by PCR. If PCR is positive, the patient has current (active) infection, chronic if it persists beyond 6 months, and needs genotype or pan-genotypic antiviral treatment.

Coagulation

• DIC: low platelets and fibrinogen, long PT and APTT, high D-dimer, schistocytes. von Willebrand disease: long bleeding time and APTT, normal PT.

Other common tests

• Diabetes: fasting glucose 7.0 mmol/L (126 mg/dL) or more; random or 2-hour 11.1 mmol/L (200 mg/dL) or more; HbA1c 48 mmol/mol (6.5%) or more.
• Thyroid: TSH is the best screening test. Exogenous thyroxine gives high T4, suppressed TSH and low thyroglobulin.
• Pancreatitis: amylase or lipase over 3 times normal (lipase more specific; level does not reflect severity).
• Urine: red cell casts (glomerulonephritis), white cell casts (pyelonephritis), fatty casts (nephrotic), muddy brown casts (ATN); sterile pyuria (renal TB, stones, partly treated UTI).

• Pleural fluid: Light criteria (fluid:serum protein over 0.5, LDH over 0.6) define an exudate. Ascites: SAAG 11 g/L or more means portal hypertension; neutrophils over 250/µL mean spontaneous bacterial peritonitis.
• Pre-analytical errors: haemolysis (false high potassium, LDH, AST), delayed separation, EDTA contamination (high K+, low Ca2+), drip-arm samples.

Remember Reticulocytes measure marrow effort; ferritin measures stores but rises with inflammation; transferrin saturation settles overload.

Exam trap HbA1c is falsely low whenever red cells die early.

Exam trap A positive anti-HCV test is not active infection; confirm with HCV RNA.

How FCPS asks about this

• Hb falls in late pregnancy with normal MCV and ferritin: physiological haemodilution.
• Reticulocytes 8%: increased marrow output (haemolysis or bleeding).
• Haemochromatosis iron pattern: high transferrin saturation and high ferritin.
• Haemolysis, pancytopenia, hepatic vein thrombosis: PNH, confirmed by CD55/CD59 flow cytometry.
• Hypertension, weakness, low potassium, alkalosis, low renin: Conn adenoma.
• Hypokalaemia, urine pH never below 5.5, stones: type 1 distal RTA; hexagonal crystals with positive nitroprusside test are cystinuria.
• Uraemic pericarditis in ESRD: urgent dialysis; low HbA1c in sickle cell is from shortened red cell life.