How to read a lab report: a guide for doctors who didn't specialize in pathology

Medical school teaches you how to order lab tests. It does not always teach you how to interpret them with the nuance that clinical practice demands. If you are a general practitioner, a surgeon, or a specialist outside pathology and internal medicine, there are lab values you see every day but may not interpret with full confidence.

This is not a textbook reference — you have UpToDate and Harrison's for that. This is a practical guide to the clinical pearls that help you read lab reports faster, catch the values that matter, and avoid the common interpretation mistakes that lead to unnecessary referrals or missed diagnoses.

CBC: beyond haemoglobin

Every doctor looks at haemoglobin first. That is correct — anaemia is common in India (over 50% of women and 25% of men are anaemic). But the CBC tells you much more than just the Hb level.

MCV (Mean Corpuscular Volume) tells you the cause. Low MCV (microcytic) points to iron deficiency or thalassemia trait — both extremely common in India. High MCV (macrocytic) suggests B12 or folate deficiency, which is increasingly seen in vegetarian patients (a large proportion of the Indian population) and in patients on metformin long-term. Normal MCV with low Hb (normocytic anaemia) suggests chronic disease, acute blood loss, or renal failure.

RDW (Red Cell Distribution Width) is underappreciated. A high RDW with low MCV strongly suggests iron deficiency over thalassemia trait (thalassemia trait typically has a normal RDW). This single value can save a patient from unnecessary haemoglobin electrophoresis.

Platelet count context matters. A platelet count of 90,000 in a dengue patient needs daily monitoring. A platelet count of 90,000 in a chronic liver disease patient might be their stable baseline. Always interpret platelets in clinical context, not just against the reference range.

The WBC differential is the first line of infection classification. Neutrophil predominance suggests bacterial infection. Lymphocyte predominance suggests viral infection. Eosinophilia in an Indian patient should trigger a mental checklist: parasitic infection (very common), allergic conditions, or drug reaction. Do not ignore eosinophil counts above 500/microlitre — parasitic infections are still endemic across most of India.

Liver panel: the pattern recognition approach

When you receive a liver function test, resist the urge to look at individual values in isolation. The pattern tells you more than any single number.

Hepatocellular pattern (ALT and AST elevated, ALP normal or mildly elevated): Think viral hepatitis, drug-induced liver injury, or alcoholic hepatitis. In India, always consider Hepatitis B and E — both are prevalent. Ask about recent medications, particularly anti-TB drugs (isoniazid, rifampicin, pyrazinamide), paracetamol, and ayurvedic medications (many contain hepatotoxic heavy metals).

Cholestatic pattern (ALP and GGT elevated, ALT/AST normal or mildly elevated): Think biliary obstruction — gallstones, pancreatic head mass, or drug-induced cholestasis. An ultrasound is your next step, not more blood tests.

Mixed pattern: Consider infiltrative diseases or a combination of pathologies, which is common in India where patients often have multiple concurrent conditions.

The AST:ALT ratio gives you a clue. An AST:ALT ratio greater than 2:1 strongly suggests alcoholic liver disease. A ratio less than 1 is typical of non-alcoholic fatty liver disease (NAFLD), which is reaching epidemic proportions in urban India due to sedentary lifestyles and dietary changes.

Albumin is the liver's report card over time. A low albumin does not mean the liver failed yesterday. It means the liver has been struggling for weeks to months. Albumin has a half-life of 21 days. It is a chronic marker, not an acute one. Do not panic about a mildly low albumin in the context of normal ALT/AST — consider nutritional deficiency first, which is extremely common in elderly Indian patients.

Renal panel: creatinine alone is not enough

Creatinine must be interpreted with muscle mass. A creatinine of 1.4 mg/dL in a 30-year-old muscular man might be normal. The same value in a 70-year-old frail woman indicates significant renal impairment. Always calculate eGFR — the lab should provide it, but if they do not, use the CKD-EPI equation. In elderly Indian patients, especially women, the creatinine "looks normal" even when eGFR is below 60.

BUN:Creatinine ratio guides you. A ratio greater than 20:1 suggests pre-renal azotemia — dehydration, heart failure, or upper GI bleed. In the Indian summer, dehydration-related pre-renal azotemia is extremely common. A BUN:Creatinine ratio of 10-15:1 is typical of intrinsic renal disease.

Uric acid is not just about gout. Elevated uric acid is associated with cardiovascular risk, metabolic syndrome, and chronic kidney disease progression. In the Indian population, where vegetarian diets rich in pulses and legumes contribute to purine load, elevated uric acid is common and should be addressed in the broader metabolic context.

Electrolytes need clinical context. A potassium of 5.3 mEq/L might be a lab error (haemolysed sample — very common in Indian labs, especially when blood draws are difficult), or it might be real hyperkalemia in a patient on ACE inhibitors with mild CKD. Before treating a high potassium, confirm with a repeat sample if there is no ECG change.

Thyroid function: the TSH-first approach

TSH is your screening test. Free T4 confirms. In most clinical scenarios, an abnormal TSH followed by Free T4 measurement gives you the diagnosis. You do not need to order T3, T4, Free T3, Free T4, and TSH simultaneously — this is wasteful and leads to confusing results.

Subclinical hypothyroidism is overtreated in India. A TSH of 6-8 mIU/L with normal Free T4 in an asymptomatic patient does not automatically need levothyroxine. Current guidelines recommend monitoring rather than immediate treatment for TSH below 10, unless the patient is pregnant, trying to conceive, or has symptoms. The tendency to start levothyroxine at TSH 5.5 is common in Indian practice and leads to lifelong unnecessary medication.

Sick euthyroid syndrome is a trap. In hospitalised patients — especially in the ICU — thyroid function tests are unreliable. Low T3, low or normal T4, and variable TSH in a sick patient do not mean hypothyroidism. They mean the patient is sick, and the thyroid axis is doing what it does during illness. Do not start thyroid medication based on labs drawn during acute illness.

Diabetes markers: beyond fasting glucose

HbA1c is your 90-day average, but it has caveats. Conditions that affect red blood cell turnover — thalassemia trait (very common in India, especially in Gujarat, Maharashtra, and parts of South India), iron deficiency anaemia, haemolytic anaemia, chronic kidney disease — can falsely lower or raise HbA1c. A patient with thalassemia trait and an HbA1c of 5.8% may actually have diabetes-range glucose levels. In such patients, fructosamine or continuous glucose monitoring is more reliable.

Fasting glucose varies with stress. A fasting glucose of 130 mg/dL in a hospitalised patient running a fever does not diagnose diabetes. Stress hyperglycemia is real and common. Confirm with a repeat fasting glucose or HbA1c after the acute illness resolves.

Post-prandial glucose matters in Indian diets. The typical Indian diet is carbohydrate-heavy — rice, roti, and potatoes are staples. Post-prandial glucose spikes are often more significant than fasting values in Indian diabetic patients. A patient with a normal fasting glucose but a 2-hour post-prandial of 220 mg/dL needs attention. Consider ordering post-prandial glucose routinely, not just fasting.

Microalbuminuria is the early warning. In diabetic patients, urine microalbumin (or urine albumin-to-creatinine ratio) detects diabetic nephropathy years before creatinine rises. Screen annually from the time of diagnosis in Type 2 diabetes. It is inexpensive, widely available, and changes management — early ACE inhibitor or ARB therapy slows progression.

The practical takeaway

Lab reports are tools, not diagnoses. Every value needs clinical context. The reference range printed on the report is a statistical range for a healthy population — your patient's "normal" may be different based on their age, sex, body composition, diet, medications, and comorbidities.

When in doubt, repeat the test before acting on it. A single abnormal value in an asymptomatic patient is often a lab error, a transient fluctuation, or a pre-analytical issue. Two abnormal values are a pattern that needs attention.

And finally, talk to your pathologist. They are not just the people who sign the report. A good pathologist can tell you whether that borderline result is clinically significant, whether the sample quality was adequate, and what additional test would clinch the diagnosis. In Indian hospitals, the pathologist is an underutilised clinical resource.