Clinical pathology

In short

Clinical pathology is the branch of laboratory medicine that examines body fluids and excreta — urine, stool, semen, cerebrospinal fluid (CSF), pleural, peritoneal and synovial fluids — by physical, chemical and microscopic methods to screen for and monitor disease. It is the first laboratory contact in almost every OPD visit in India: a urine routine and microscopy (R/M) is among the most frequently ordered tests, and stool and body-fluid reports often decide admission or surgery. The discipline also owns the pre-analytical chain — patient identification, phlebotomy, specimen collection, transport, rejection criteria — plus internal quality control, NABL/ISO 15189 documentation and biomedical-waste handling, which is why it dominates B.Sc MLT papers and technician interviews. Employers (Dr Lal PathLabs, Metropolis, SRL, Thyrocare, Apollo, AIIMS/PGIMER, state medical colleges, ESIC and district hospitals) test it in practical exams, DMLT/B.Sc viva and placement technical rounds.

Explained

Scope, specimens and the pre-analytical chain

• Clinical pathology covers routine and special examination of body fluids and excreta; in Indian hospital structure it sits alongside haematology, biochemistry, microbiology and histopathology as a separate reporting bench.
• Three levels of examination for any fluid: physical (volume, colour, appearance, specific gravity/turbidity), chemical (dipstick or wet chemistry), microscopic (wet mount, centrifuged deposit, stained smear).
• Pre-analytical errors cause the majority of laboratory errors (commonly quoted as roughly 60-70% in literature; the exact share varies by study and lab). Causes: wrong patient, wrong container, wrong preservative, under-filled or over-filled tube, haemolysis, delay in transport, no fasting.
• Order of draw (CLSI): blood culture bottle → citrate (light blue, coagulation) → plain/serum or gel SST (red/yellow) → heparin (green) → EDTA (lavender) → fluoride-oxalate (grey). Prevents additive carry-over (EDTA before green falsely lowers calcium and raises potassium).
• Vacutainer additives: EDTA chelates calcium (haematology, HbA1c), sodium citrate 3.2% in 1:9 ratio (PT/INR, APTT), lithium heparin (plasma chemistry, ammonia, lactate), sodium fluoride + potassium oxalate (glucose, inhibits glycolysis), plain/clot activator (serum).
• Specimen rejection criteria: unlabelled or mismatched, clotted EDTA sample, haemolysed sample for potassium/LDH, under-filled citrate tube, wrong tube, leaked or contaminated container, excessive transport delay. Rejection must be documented and the clinician informed — an NABL requirement.
• Universal/standard precautions: treat every specimen as infectious; gloves, lab coat, face protection for aerosol work, no mouth pipetting, no recapping needles, hand hygiene, immediate spill decontamination with 1% hypochlorite (freshly prepared; 10,000 ppm available chlorine for spills, 0.5-1% for surfaces).

Urine examination (urinalysis)

• Collection types: random (screening), early morning first-void (most concentrated — best for protein, cells, casts, nitrite, pregnancy test), midstream clean-catch (culture), 24-hour (protein, creatinine clearance, VMA, calcium), catheter/suprapubic (sterile), paediatric urine bag.
• Preservatives: refrigeration 2-8 °C is best for routine (examine within 2 hours ideally); boric acid for culture; toluene or thymol for chemical analysis; 6 N HCl for VMA/metanephrines and calcium; sodium carbonate for urobilinogen; formalin to preserve cells and casts for microscopy.
• Physical examination: volume (normal adult 800-2,000 mL/day; polyuria >2.5-3 L, oliguria <400 mL, anuria <100 mL), colour (pale — dilute/diabetes; dark yellow — concentrated; red — haematuria, beeturia, rifampicin; cola/brown — myoglobin, bilirubin, alkaptonuria; milky — chyluria in filariasis), appearance (clear, turbid, frothy in heavy proteinuria), odour (ammoniacal on standing, fruity/acetone in ketosis).
• Specific gravity 1.003-1.030 (urinometer, refractometer or dipstick). Fixed SG about 1.010 (isosthenuria) suggests loss of concentrating ability in chronic renal failure. Osmolality 50-1,200 mOsm/kg.
• pH 4.5-8.0 (average about 6.0). Alkaline urine: urea-splitting *Proteus* UTI, vegetarian diet, renal tubular acidosis, old sample. Acidic urine: ketoacidosis, high protein diet.
• Dipstick (reagent strip) pads and chemistry:
  • Protein — protein-error-of-indicator (tetrabromophenol blue); detects mainly albumin, misses Bence Jones protein and is negative in microalbuminuria.
  • Glucose — glucose oxidase/peroxidase, specific for glucose only.
  • Ketone — nitroprusside (sodium nitroprusside) detects acetoacetate strongly, acetone weakly, not β-hydroxybutyrate (the main ketone in DKA).
  • Blood — pseudoperoxidase of haem; positive with intact RBCs, free haemoglobin and myoglobin (so a positive blood pad with no RBCs on microscopy suggests haemolysis or rhabdomyolysis).
  • Nitrite — Griess reaction, positive with nitrate-reducing Gram-negatives; negative with *Enterococcus*, *Staphylococcus saprophyticus*, *Pseudomonas*.
  • Leucocyte esterase — detects granulocyte esterase, positive even in lysed WBCs; nitrite + LE together form the standard UTI screen.
  • Bilirubin (diazo), urobilinogen (Ehrlich), urine SG (polyelectrolyte), pH (double indicator).
• Confirmatory chemical tests: heat-and-acetic-acid or sulphosalicylic acid for protein (SSA detects all proteins including Bence Jones), Benedict's test for reducing sugars (green→yellow→orange→brick red, so it also turns positive in lactosuria, galactosuria, fructosuria, high-dose vitamin C), Rothera's test for ketones, Fouchet's test for bilirubin, Ehrlich's aldehyde for urobilinogen, Bence Jones protein by heating to 40-60 °C (precipitates) and clearing at 100 °C — now replaced by serum/urine electrophoresis and free light chains.
• Microscopy of centrifuged deposit (10-12 mL at about 1,500-2,000 rpm for 5 minutes, decant, resuspend, examine under low and high power):
  • RBCs: normal 0-2/HPF; dysmorphic RBCs and RBC casts indicate glomerular bleeding; isomorphic RBCs suggest stones, trauma, tumour, TB.
  • WBCs (pus cells): normal 0-5/HPF; >5-10/HPF = pyuria; sterile pyuria in renal TB, treated UTI, interstitial nephritis, analgesic nephropathy.
  • Epithelial cells: squamous (contamination), transitional, renal tubular epithelial cells (acute tubular necrosis).
  • Casts (formed in tubules, Tamm-Horsfall protein matrix): hyaline (normal, fever, exercise), RBC (glomerulonephritis), WBC (pyelonephritis), granular (tubular damage), epithelial cell (ATN), waxy and broad casts (chronic renal failure, "renal failure cast"), fatty casts with oval fat bodies and Maltese-cross appearance (nephrotic syndrome).
  • Crystals: calcium oxalate (envelope/dumbbell, acidic), uric acid (rhomboid/rosette, acidic), triple phosphate (coffin-lid, alkaline), cystine (hexagonal, cystinuria), cholesterol (notched plates), bilirubin, sulphonamide and acyclovir drug crystals.
  • Others: yeast, *Trichomonas vaginalis* (motile), *Schistosoma haematobium* ova (terminal spine), spermatozoa, mucus threads, artefacts (fibres, starch, air bubbles, talc).
• Automation: semi-automated strip readers and automated urine particle analysers (flow-cytometry or digital imaging) are standard in large Indian chains; abnormal flagged samples still need manual microscopy.
• Pregnancy test: urine hCG by immunochromatographic card using anti-α and anti-β-hCG antibodies; sensitivity typically 10-25 mIU/mL; first morning sample best. False negative — very early pregnancy, dilute urine, hook effect in very high hCG (molar pregnancy); false positive — hCG-secreting tumours, recent delivery/abortion, some assays with high LH.

Stool (faeces) examination

• Collection: fresh, in a clean wide-mouth container, no urine or water contamination; examine within 30-60 minutes for motile trophozoites (keep warm); preservatives — 10% formalin (ova, cysts), PVA (trophozoite staining), SAF, MIF.
• Physical: colour (pale/clay — biliary obstruction and steatorrhoea; black tarry melaena — upper GI bleed, also iron/bismuth; red — lower GI bleed; rice-watery — cholera; "anchovy sauce" — amoebic dysentery (red-currant jelly stool is classically intussusception)), consistency, mucus, blood, visible worms or segments.
• Microscopy: saline wet mount (motility, RBC, pus cells, trophozoites), iodine mount (cyst nuclei and glycogen), concentration methods — formol-ether (formalin-ethyl acetate) sedimentation and zinc sulphate/saturated salt flotation to raise sensitivity.
• Findings: cysts and trophozoites of *Entamoeba histolytica* (ingested RBCs in trophozoite), *Giardia lamblia* (pear-shaped, falling-leaf motility), helminth ova (*Ascaris*, hookworm, *Trichuris*, *Taenia*, *Hymenolepis*), *Strongyloides* larvae, Charcot-Leyden crystals (eosinophil breakdown), undigested muscle fibres and fat globules (malabsorption), Sudan III stain for fat.
• Special stains: modified Ziehl-Neelsen (modified acid-fast) for *Cryptosporidium*, *Cyclospora*, *Isospora/Cystoisospora* — important in HIV diarrhoea; trichrome stain for protozoa.
• Occult blood: guaiac-based (influenced by diet, meat, peroxidase-rich vegetables, vitamin C) versus faecal immunochemical test (FIT) using antibody to human globin — more specific, no dietary restriction, used for colorectal cancer screening.
• Other stool tests: reducing substances (Benedict's) for disaccharidase deficiency, stool pH (<5.5 in carbohydrate malabsorption), faecal calprotectin for intestinal inflammation, *H. pylori* stool antigen, rotavirus antigen, *C. difficile* toxin, scotch-tape/perianal swab for *Enterobius vermicularis* eggs.

Semen analysis and other secretions

• Collection: after 2-7 days abstinence, by masturbation into a sterile wide-mouth non-toxic container, complete sample, kept at 20-37 °C, delivered within 1 hour; two or three samples 2-4 weeks apart before concluding. Record time of collection and any loss of the first fraction (sperm-rich).
• Macroscopic: liquefaction within 15-30 minutes (semenogelin breakdown by PSA), volume (lower reference limit about 1.4-1.5 mL per WHO 6th edition, 2021), appearance grey-opalescent, viscosity, pH 7.2 or above.
• Microscopic: concentration (Neubauer chamber after dilution; lower reference limit about 16 million/mL and total count about 39 million per ejaculate, WHO 6th edition), total motility about 42% and progressive motility about 30%, normal morphology about 4% (strict Kruger criteria, Papanicolaou/Giemsa stain), vitality by eosin-nigrosin, round cells and leucocytes (<1 million/mL), agglutination, MAR/immunobead test for antisperm antibodies.
• Terminology: azoospermia (no sperm — confirm on centrifuged deposit), oligozoospermia, asthenozoospermia (motility), teratozoospermia (morphology), oligoasthenoteratozoospermia (OAT), necrozoospermia, aspermia (no ejaculate), post-vasectomy semen analysis for azoospermia confirmation.
• Note: WHO reference limits are the 5th centile of fertile men, not a cut-off between fertile and infertile; values below them do not prove infertility.
• Sex determination and any prenatal sex selection is prohibited in India under the PC-PNDT Act, 1994 (amended 2003) — relevant to all reproductive-laboratory work.
• Sputum (clinical pathology side): volume, consistency, colour, blood; microscopy for eosinophils, Curschmann spirals, Charcot-Leyden crystals in asthma; AFB and GeneXpert belong to microbiology.
• Gastric/duodenal aspirate: pH, occult blood, AFB in children unable to expectorate.

CSF and serous body fluids

• CSF collection: lumbar puncture at L3-L4/L4-L5 by a clinician; three or four sterile tubes — tube 1 chemistry/immunology, tube 2 microbiology, tube 3 cell count and cytology (least contaminated by needle blood), extra tube for special studies; process immediately — cells lyse within about an hour; never refrigerate the microbiology tube (meningococcus dies).
• Normal CSF: clear, colourless, "crystal clear"; pressure 70-180 mm H₂O; volume about 150 mL in adults; cells 0-5 lymphocytes/µL (neonates up to about 30); protein 15-45 mg/dL; glucose 45-80 mg/dL (about 60-70% of plasma glucose); chloride 115-130 mEq/L.
• Cell count in an undiluted sample using a Neubauer chamber (all 9 large squares if cells are few); differential on a cytocentrifuged, stained smear.
• Classic patterns:

• Xanthochromia (yellow supernatant after centrifugation) indicates subarachnoid haemorrhage, not a traumatic tap; the three-tube test (blood clearing from tube 1 to tube 3) also favours a traumatic tap.
• Serous fluids (pleural, peritoneal/ascitic, pericardial): collect in EDTA (cell count), plain and fluoride tubes (chemistry), sterile container and culture bottles (microbiology), and in fixative/plain for cytology.
• Transudate vs exudate — Light's criteria (exudate if any one is met): fluid/serum protein >0.5, fluid/serum LDH >0.6, fluid LDH > two-thirds of the upper limit of normal serum LDH. Transudate: cardiac failure, cirrhosis, nephrotic syndrome, hypoalbuminaemia. Exudate: infection, TB, malignancy, pancreatitis, pulmonary embolism, collagen disease.
• Supporting tests: fluid ADA and lymphocytosis for TB pleural effusion (common in India), fluid glucose and pH for complicated effusion/empyema, amylase for pancreatic effusion, triglycerides for chylothorax, SAAG (serum-ascites albumin gradient) ≥1.1 g/dL for portal hypertension, ascitic PMN count ≥250/µL for spontaneous bacterial peritonitis.
• Synovial fluid: viscosity and string sign (hyaluronate), cell count and differential, polarised-light microscopy — needle-shaped negatively birefringent monosodium urate crystals (gout) versus rhomboid weakly positively birefringent calcium pyrophosphate crystals (pseudogout); turbid, high-neutrophil fluid with positive Gram stain in septic arthritis.

Quality, safety, regulation and records

• Quality control: run two or three levels of control material with each batch; plot on a Levey-Jennings chart and apply Westgard rules (1₃s, 2₂s, R₄s, 4₁s, 10x) — a violated rule means reject the run, investigate, do not report. External quality assurance (EQAS/PT) from CMC Vellore, AIIMS, BALQS or NABL-approved providers.
• Precision, accuracy, bias: precision from repeated runs (SD, CV%), accuracy against assigned value; CV% is the comparable index across analyte levels.
• Sensitivity and specificity govern screening decisions (dipstick nitrite is specific but insensitive; leucocyte esterase is sensitive but less specific).
• Accreditation: NABL as per ISO 15189 (medical laboratory quality and competence) — mandatory elements include SOPs for every test, equipment calibration and maintenance logs, reagent lot records, temperature logs for refrigerators and incubators, personnel competency assessment, turnaround-time monitoring, critical-value call-out register, and a documented sample rejection policy. NABL 112 is the specific application document for medical testing laboratories.
• Clinical Establishments (Registration and Regulation) Act, 2010 governs registration of laboratories in states that have adopted it; many states run their own clinical-establishment rules.
• Biomedical Waste Management Rules, 2016 (as amended): Yellow — human/animal tissue, soiled waste, laboratory and microbiological waste, discarded medicines (incineration/deep burial); Red — contaminated recyclable plastics such as tubing, syringes without needles, urine containers, gloves (autoclave/microwave then shred); White (translucent, puncture-proof) — needles, sharps, blades; Blue — broken/discarded glass and metallic implants (disinfect, then recycle). Pre-treat laboratory and blood-bag waste by autoclaving/disinfection before disposal; maintain a waste register and annual return to the State Pollution Control Board.
• Other legal context: HIV and AIDS (Prevention and Control) Act, 2017 — informed consent and confidentiality for HIV testing; PC-PNDT Act, 1994 for sex determination; Atomic Energy Regulatory Board rules for radioimmunoassay waste where used; Drugs and Cosmetics rules cover diagnostic kits (in-vitro diagnostics are notified as drugs).
• Reporting discipline: report with units and reference interval, flag critical values and telephone them with documentation, never alter a released report — issue an amended report with reason and signature, retain records (commonly a minimum of 1-3 years for reports and longer for histopathology blocks/slides, per NABL and institutional policy).

Key points

• Clinical pathology - physical, chemical and microscopic examination of urine, stool, semen, CSF and serous fluids.
• Early morning midstream sample - best for routine urinalysis; examine within 2 hours or refrigerate at 2-8 °C.
• Specific gravity 1.003-1.030 - fixed 1.010 (isosthenuria) means lost concentrating power in chronic renal failure.
• Dipstick protein pad - detects albumin only; misses Bence Jones protein and microalbuminuria, so use SSA or an albumin-specific assay.
• Ketone pad (nitroprusside) - detects acetoacetate, not β-hydroxybutyrate, so it underestimates early DKA.
• Nitrite + leucocyte esterase - the dipstick UTI screen; nitrite is negative with *Enterococcus* and *Pseudomonas*.
• Blood pad positive with no RBCs - free haemoglobin or myoglobin (haemolysis, rhabdomyolysis).
• RBC casts and dysmorphic RBCs - glomerular bleeding; waxy/broad casts - chronic renal failure.
• Maltese cross with oval fat bodies - lipiduria of nephrotic syndrome.
• Coffin-lid triple phosphate crystals - alkaline urine, often urea-splitting *Proteus*.
• Formol-ether concentration - raises sensitivity of stool examination for ova and cysts; iodine mount shows cyst nuclei.
• Modified ZN stain - *Cryptosporidium*, *Cyclospora*, *Cystoisospora* in HIV-associated diarrhoea.
• FIT over guaiac - antibody to human globin, no dietary restriction, better specificity for occult blood.
• WHO 6th edition semen limits - volume about 1.4 mL, concentration about 16 million/mL, total motility about 42%, normal forms about 4%; these are 5th centiles, not fertility cut-offs.
• CSF three or four tubes - cell count from the last tube; process at once because cells lyse within an hour.
• Light's criteria - protein ratio >0.5, LDH ratio >0.6, or fluid LDH > two-thirds upper normal serum LDH, identify an exudate.
• SAAG ≥1.1 g/dL - portal hypertensive ascites; ascitic PMN ≥250/µL - spontaneous bacterial peritonitis.
• Westgard rules on a Levey-Jennings chart - 1₃s or 2₂s violation means reject the run; NABL/ISO 15189 and BMW Rules 2016 govern the lab's records and waste.

Formulas and examples

Example 1 — CSF cell count. Undiluted CSF, 36 cells counted in all 9 large squares of a Neubauer chamber.

• Volume counted = 9 × 1 mm² × 0.1 mm = 0.9 µL.
• Cells/µL = 36 / 0.9 = 40 cells/µL, dilution factor 1.
• Differential shows 85% lymphocytes with protein 180 mg/dL and glucose 28 mg/dL (plasma 96 mg/dL) → lymphocytic pleocytosis, high protein, low glucose = pattern of tubercular meningitis; send for ZN stain, CBNAAT and ADA.

Example 2 — 24-hour urine protein. 24-hour volume 1,800 mL, protein concentration 220 mg/dL.

• Protein = 220 × 1,800 / 100 = 3,960 mg/day = 3.96 g/day.
• Above 3.5 g/day → nephrotic-range proteinuria; expect dipstick 3+ to 4+, lipiduria and fatty casts.

Example 3 — creatinine clearance. Urine creatinine 72 mg/dL, 24-h volume 1,500 mL, serum creatinine 1.8 mg/dL.

• Urine creatinine excreted = 72 × 1,500 / 100 = 1,080 mg/day.
• CrCl = (72 × 1,500) / (1.8 × 1,440) = 108,000 / 2,592 = 41.7 mL/min → moderately reduced; check that collection was complete (adult creatinine output is roughly 20-25 mg/kg/day for men and 15-20 mg/kg/day for women).

Example 4 — semen analysis. Volume 2.0 mL; after 1:20 dilution, 40 sperm counted in 2 large WBC squares of a Neubauer chamber; total motility 35%, progressive 22%, normal forms 3%.

• Volume counted = 2 × 1 mm² × 0.1 mm = 0.2 µL.
• Concentration = (40 / 0.2) × 20 = 4,000 sperm/µL = 4 million/mL.
• Total count = 4 × 2.0 = 8 million per ejaculate.
• Interpretation: below WHO 6th-edition lower limits on count, motility and morphology → oligoasthenoteratozoospermia; repeat after 2-4 weeks before reporting a conclusion.

Example 5 — pleural fluid, Light's criteria. Fluid protein 4.2 g/dL, serum protein 6.8 g/dL; fluid LDH 420 U/L, serum LDH 240 U/L (serum upper normal 250 U/L).

• Protein ratio = 4.2 / 6.8 = 0.62 (>0.5 — met).
• LDH ratio = 420 / 240 = 1.75 (>0.6 — met).
• Two-thirds of upper normal serum LDH = 0.667 × 250 ≈ 167 U/L; fluid LDH 420 > 167 (met).
• Exudate. With lymphocyte predominance and raised ADA in an Indian adult, tubercular effusion is the leading diagnosis.

Where it is used

• Diagnostic chains and hospital labs: Dr Lal PathLabs, Metropolis, SRL, Thyrocare, Apollo, Fortis, Max and medical-college central labs run high-volume urine R/M, stool, CSF and body-fluid benches; a B.Sc MLT graduate typically starts as a technician on the clinical pathology/phlebotomy bench.
• Government service: district hospitals, CHC/PHC laboratories under NHM, ESIC, railway and defence hospitals; Free Diagnostics Service Initiative essential diagnostics lists make urine routine, stool and pregnancy testing mandatory at primary level.
• Public-health programmes: national TB elimination programme (specimen handling), filaria and soil-transmitted-helminth surveys using stool and urine examination, maternal health programmes using urine protein screening for pre-eclampsia.
• Job roles: lab technician/technologist, phlebotomist, sample-collection and logistics coordinator, quality officer for NABL documentation, application specialist for urine analyser and point-of-care manufacturers (Siemens Healthineers, Roche, Sysmex, Transasia/Erba, Mindray, Agappe, Arkray distributors).
• Fertility and urology practice: IVF and andrology labs (semen analysis, post-vasectomy checks, sperm preparation), dialysis units (urine output and clearance monitoring), nephrology OPDs (UPCR monitoring).
• Pre-employment, insurance and fitness screening: urine routine and drugs-of-abuse screening for corporates, visa and transport medicals; occupational health labs reporting under employer health-surveillance programmes.

Interview questions

• Q: Why is an early morning midstream sample preferred for urine routine examination? A: It is the most concentrated, so formed elements, protein, nitrite and hCG are easier to detect, and the midstream portion avoids urethral and genital contamination.
• Q: A dipstick shows blood 2+ but microscopy shows no RBCs — explain. A: The pad detects haem pseudoperoxidase, so free haemoglobin (intravascular or in-vitro haemolysis) or myoglobin (rhabdomyolysis) gives a positive result without intact cells.
• Q: Which protein does the dipstick miss and how do you detect it? A: Bence Jones protein (free light chains) and low-level microalbuminuria; use sulphosalicylic acid or heat precipitation for screening and serum/urine electrophoresis or free light chain assay to confirm.
• Q: Why is a nitrite-negative dipstick not enough to rule out UTI? A: Organisms like *Enterococcus*, *Staphylococcus saprophyticus* and *Pseudomonas* do not reduce nitrate, and a short bladder dwell time or low dietary nitrate also gives a false negative; confirm with culture.
• Q: What does a waxy or broad cast mean? A: It forms in dilated, poorly functioning tubules and indicates advanced chronic renal disease — hence the name renal failure cast.
• Q: Name the CSF tube used for cell count and why. A: The last tube (tube 3 or 4), because any blood from needle trauma clears progressively and this tube least reflects the traumatic tap.
• Q: How do you distinguish a traumatic tap from subarachnoid haemorrhage? A: In a traumatic tap blood decreases from tube 1 to tube 3 and the centrifuged supernatant is clear; in subarachnoid haemorrhage blood is uniform and the supernatant is xanthochromic.
• Q: State Light's criteria and their purpose. A: Fluid/serum protein >0.5, fluid/serum LDH >0.6, or fluid LDH above two-thirds of the upper normal serum LDH; any one makes the effusion an exudate, which directs investigation towards infection, TB or malignancy.
• Q: Why must a citrate tube be filled exactly to the mark? A: The 1:9 anticoagulant-to-blood ratio must hold; under-filling leaves excess citrate that binds added calcium and falsely prolongs PT and APTT.
• Q: What is the correct order of draw and why does it matter? A: Blood culture, citrate, serum, heparin, EDTA, fluoride-oxalate — to prevent additive carry-over, for example EDTA contamination falsely lowering calcium and raising potassium.
• Q: Which Westgard rule rejects a run outright? A: 1₃s (a single control beyond ±3 SD) and 2₂s (two consecutive controls beyond the same ±2 SD limit) indicate random and systematic error respectively; the run is rejected, investigated and repeated before reporting.
• Q: Which bin takes a used urine container and which takes a needle, under the 2016 rules? A: The contaminated plastic container goes in the red bag for autoclaving and shredding; the needle goes in the white translucent puncture-proof container for sharps.

Common mistakes

• Reporting a delayed urine sample as normal — casts dissolve, RBCs lyse, bacteria multiply and pH rises on standing, so the microscopy becomes unreliable.
• Treating a positive Benedict's test as glycosuria; it detects any reducing sugar and is positive in lactosuria, galactosuria and with high-dose vitamin C.
• Calling a dipstick-negative ketone result "no ketosis" in diabetic ketoacidosis, when nitroprusside does not detect β-hydroxybutyrate.
• Forgetting the chamber depth of 0.1 mm or the dilution factor in cell and sperm counts, giving answers that are out by a factor of 10 or 20.
• Reporting squamous epithelial cells and contaminant WBCs from a non-midstream female sample as pyuria instead of recollecting.
• Concluding infertility or azoospermia from a single semen sample, or from an incomplete or delayed specimen that lost the sperm-rich first fraction.
• Mixing up the biomedical waste colour codes (sharps into red, or untreated laboratory waste into general waste) and skipping the temperature, calibration and QC logs that NABL assessors check first.

Related topics

Haematology · Clinical biochemistry: organ function tests · Medical microbiology: infections and diagnosis · Histopathology and cytology · Immunology and serology tests · Blood banking and transfusion