# Laboratory Investigation Basics

> Laboratory investigations for BDS General Surgery — haemogram, PT INR versus aPTT, renal and liver function, blood sugar, cultures and CBNAAT for TB.

- Canonical URL: https://prepelephant.com/topics/bds/general-surgery/lab-investigation-basics-bds
- Exam / course: BDS · Subject: General Surgery
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Laboratory Investigation Basics", PrepElephant, https://prepelephant.com/topics/bds/general-surgery/lab-investigation-basics-bds

## Direct answer

Order a test only when its result will change management — the discipline that separates investigation from decoration. The surgical baseline reads: haemoglobin (roughly 13-17 g/dL in men, 12-15 in women), total leucocyte count (4,000-11,000 per microlitre, rising with bacterial infection), platelet count (150,000-450,000), and the coagulation pair — prothrombin time with INR reflecting the extrinsic pathway and warfarin effect, activated partial thromboplastin time reflecting the intrinsic pathway and heparin effect. Renal function is read through urea and creatinine (creatinine around 0.7-1.3 mg/dL), liver function through bilirubin, transaminases, alkaline phosphatase (also a bone enzyme — raised in Paget disease and bone metastases) and albumin, and glycaemic status through fasting glucose and HbA1c (6.5 per cent or more supports diabetes). Microbiology answers what is growing: pus, urine and blood cultures taken before antibiotics, acid-fast staining and CBNAAT for tuberculosis under the national programme, and the NACO three-test algorithm for HIV.

## What you must remember

- **Haemogram with surgical meaning:** anaemia thresholds — men below 13 g/dL, women below 12 g/dL (WHO); leucocytosis with neutrophilia in bacterial infection and leucopenia in viral illness and typhoid; platelets below 150,000 as thrombocytopenia with bleeding risk in surgery roughly below 50,000; ESR and C-reactive protein as nonspecific inflammation trackers.
- **Coagulation pair by pathway:** PT/INR — extrinsic (factor VII first to fall in vitamin K deficiency and liver failure; monitors warfarin); aPTT — intrinsic (prolonged in haemophilia and on heparin); both prolonged with low platelets in DIC — the pattern recognition exam.
- **Renal numbers:** urea and creatinine rise together in dehydration and renal failure, creatinine the more specific; a rising creatinine postoperously or post-contrast demands review of nephrotoxic drugs (NSAIDs, aminoglycosides, contrast media).
- **Liver panel logic:** bilirubin (jaundice clinically visible above about 2-3 mg/dL), ALT and AST for hepatocellular injury, alkaline phosphatase for cholestasis and bone disease, albumin and prothrombin as synthetic-function tests — low albumin predicts poor wound healing.
- **Glycaemic panel:** fasting plasma glucose 70-110 mg/dL, random over 200 with symptoms diagnostic, HbA1c 6.5 per cent or more diagnostic of diabetes and every-3-months monitoring — uncontrolled diabetes (commonly quoted above about 200-250 mg/dL random) postpones elective surgery and impairs healing.
- **Microbiology discipline:** pus culture before antibiotics (aerobic and anaerobic transport), two blood culture sets in fever with rigors, urine culture with midstream sample, Gram stain the same day; for tuberculosis — sputum or aspirate for AFB and CBNAAT (cartridge-based nucleic acid amplification), which also detects rifampicin resistance and drives regimen choice under NTEP.
- **Serology screen for surgery and exposure:** HIV by the NACO three-test strategy, HBsAg, anti-HCV — preoperative screening and post-exposure prophylaxis decisions for the dental surgeon both hang on these.
- **Histopathology remains the gold standard** for tumours — cytology screens, biopsy diagnoses, and immunohistochemistry characterises; no tumour marker replaces tissue.

## Reading a panel like a story: three preoperative reports

Three patients, three panels, three decisions. A 45-year-old woman for multiple extractions: haemoglobin 9.8 with low MCV — iron deficiency until proven otherwise; the operation is postponed a fortnight for oral iron and a cause hunt (menorrhagia, hookworm in endemic districts), because anaemia impairs oxygen delivery, healing and local anaesthesia tolerance. A 60-year-old man on warfarin after a valve replacement: INR 3.2 — the plan is made with the cardiologist (pause, bridge or proceed with local haemostatic measures per procedure risk, INR targets documented), not by improvisation at the chair. A young man with a neck abscess: total count 18,000 with neutrophilia, glucose 320 mg/dL new — the leucocytosis is expected, the diabetes is the finding that changes everything: drainage still today, but insulin, cultures, and a long conversation about foot and dental care follow. In each story the number mattered only because someone had decided in advance what a high or low value would trigger — the entire skill of investigation is the question asked before the test.

## How the exam frames it

Short notes demanded by Indian universities: haemogram values, PT and aPTT with their pathways, liver function tests and their interpretation, and investigations in a patient with a neck swelling — where the answer runs ultrasound, FNAC, then targeted panels. MCQs mine the numbers: anaemia cutoffs, platelet thresholds, HbA1c diagnostic level, alkaline phosphatase as a bone enzyme. The Indian-practice payload is programme testing — CBNAAT for tuberculosis under NTEP and the NACO HIV testing algorithm, both quotable as national-programme details that examiners reward.

## Frequently asked questions

### What do PT/INR and aPTT each measure?

PT/INR assesses the extrinsic and common pathways (factor VII first affected, used to monitor warfarin), while aPTT assesses the intrinsic and common pathways (prolonged in haemophilia and during heparin therapy).

### At what haemoglobin is elective surgery generally deferred for anaemia correction?

Commonly below 8-10 g/dL depending on procedure and comorbidity — with cause sought and iron replaced, since anaemia impairs healing and oxygen delivery; thresholds vary by guideline.

### Why is HbA1c used alongside blood glucose?

It reflects average glycaemia over about three months, confirms the diagnosis at 6.5 per cent or more, and gauges long-term control before elective surgery better than any single glucose value.

### What is CBNAAT and its programme role in India?

Cartridge-based nucleic acid amplification testing for Mycobacterium tuberculosis and rifampicin resistance, deployed under the National Tuberculosis Elimination Programme for rapid diagnosis from sputum or aspirated samples.

### Which tests make up the standard preoperative viral screen?

HIV (by the NACO three-test strategy), hepatitis B surface antigen and anti-HCV — protecting patient management decisions and occupational post-exposure protocols alike.