Clinical Pathology for NEET PG: Urine Casts, Body Fluids (Light's Criteria, SAAG, CSF), Semen Analysis WHO 2021, LFT, RFT, TFT

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🎯 Exam priority: High. Expect: which urinary cast means what, the definitions of haematuria and nephrotic-range proteinuria, KDIGO AKI and prerenal indices, Light's criteria, SAAG and SBP, CSF patterns, WHO semen values, the AST:ALT ratio in alcohol, and TSH-based thyroid patterns.

CBME mapping: PA 22.1–22.5. Urine and the semen/thyroid panel are hands-on skills; the liver and kidney panels are the two that need certification.

Urine, pleural, ascitic and CSF findings with their meaning, and WHO 2021 semen values. Drawn by TrackNation.

Urine: Dipstick and Sediment

• Dipstick: specific gravity 1.001–1.035; nitrites are not normally present and mean bacteriuria; leukocyte esterase comes from lysed neutrophils; glucose spills when blood glucose exceeds ~180 mg/dL. Intact red cells give a speckled pattern, free haemoglobin or myoglobin a uniform colour.

• Haematuria = > 3 RBCs per high-power field. Red urine without RBCs: haemoglobin, myoglobin, porphyria, beets, some drugs. Glomerular RBCs are small and dysmorphic. Pyuria = > 5 WBCs/HPF.

• Proteinuria: normal up to 150 mg/day; albumin 30–300 mg/day = moderately increased albuminuria (old "microalbuminuria"); > 3 g/day with oedema and low albumin = nephrotic syndrome. Tubular proteinuria is mostly light chains. Orthostatic proteinuria is benign, mostly in children and adolescents.

• Casts are Tamm-Horsfall protein moulds from the thick ascending limb, so they prove a renal origin: hyaline (non-specific, even normal); RBC casts (virtually pathognomonic of glomerulonephritis); WBC casts (pyelonephritis); muddy brown granular (acute tubular necrosis); waxy (advanced CKD); fatty casts and oval fat bodies with Maltese crosses (nephrotic syndrome).

• Nephritic urine: haematuria, dysmorphic RBCs, often RBC casts, variable protein.

Kidney Function Tests

• Creatinine clearance overestimates GFR by 10–20% (creatinine is also secreted by tubules).

• KDIGO AKI: creatinine up ≥ 0.3 mg/dL in 48 h, or ≥ 1.5× baseline within 7 days, or urine < 0.5 mL/kg/h for 6 h.

• Prerenal vs tubular injury: urine Na < 10 vs > 40 mmol/L; FENa < 1% vs > 1%; BUN:creatinine > 20 vs < 10.

• CKD: eGFR < 60 mL/min/1.73 m² for ≥ 3 months is the most reliable evidence; small kidneys on ultrasound usually mean CKD.

Body Fluids

• Pleural fluid — Light's criteria (any one = exudate): fluid:serum protein ≥ 0.5, fluid:serum LDH ≥ 0.6, fluid LDH ≥ 2/3 of the serum upper limit. Commonest transudates: heart failure, cirrhosis, nephrotic syndrome.

• ADA > 40 U/L in pleural fluid: about 95% sensitive and specific for tuberculous pleuritis (can also rise in cancer). Milky fluid high in triglycerides = chylothorax (thoracic duct). Haemothorax = fluid haematocrit > 50% of blood. A parapneumonic effusion with pH < 7.20 or glucose < 60 mg/dL needs drainage.

• Ascitic fluid: SAAG = serum albumin − ascitic albumin; ≥ 1.1 g/dL with low protein points to portal hypertension. Neutrophils > 250/µL = spontaneous bacterial peritonitis; bloody fluid suggests tumour or TB.

• CSF: a CSF glucose ≤ 18 mg/dL or CSF:blood glucose < 0.23 strongly suggests bacterial meningitis.

Semen Analysis (WHO 2021)

• The semen gel (from the seminal vesicles) is liquefied by prostatic proteases, usually within 30 minutes; examination should start within 30, at most 60, minutes. WHO's fertile reference men had abstained for 2–7 days.

• Terms: oligozoospermia = too few sperm; azoospermia = none, and only after no sperm are found in a centrifuged sample.

• pH < 7.2 suggests lack of alkaline seminal-vesicle fluid (or urine contamination); fructose shows at least one ejaculatory duct is patent.

• WHO 2021 gives the values below as the 5th centile of fertile men and warns they are not decision limits for infertility.

Liver and Thyroid Panels

• LFT: aminotransferases rise with liver-cell injury, ALP with cholestasis. AST:ALT is usually < 1 but > 2 in alcohol-related liver disease. GGT confirms a hepatic source of raised ALP (it does not rise with bone disease, childhood or pregnancy); raised GGT with normal ALP is seen in alcohol-associated hepatitis. Synthetic function = PT/INR and albumin (half-life ~20 days, so it changes slowly). Unconjugated bilirubin is water-insoluble and never appears in urine.

• TFT: TSH is the best screening test. Primary hypothyroidism: high TSH, low T4/T3. Subclinical hypothyroidism: high TSH, normal free T4. Primary hyperthyroidism: undetectable TSH; T3 toxicosis = only T3 raised. Central hypothyroidism: the pituitary makes too little TSH.

Traps the Examiner Sets

• RBC casts = glomerulonephritis; muddy brown = ATN; waxy = advanced CKD; WBC casts = pyelonephritis.

• Dipstick positive for blood but no RBCs = haemoglobin or myoglobin.

• Light's ratios: protein 0.5, LDH 0.6. SAAG 1.1. SBP > 250 neutrophils.

• WHO 2021 concentration 16 million/mL; 39 million is the total per ejaculate — do not swap them.

• AST:ALT > 2 = alcohol.

Memory Hooks & Shortcuts

Shortcuts to lock in this article's numbers and lists. Each one only re-packages facts proven above.

• 🧠 Red casts, Glomerulus Red-handed — RBC casts = glomerulonephritis

• 🧠 Muddy Brown = Many dead tubules — granular casts = ATN

• 🧠 WAXY = Wasted kidney — waxy casts = advanced CKD

• 🧠 Light is half and six-tenths — Light's ratios 0.5 and 0.6

• 🧠 SAAG 1.1 = Portal high — SAAG ≥ 1.1 = portal hypertension

• 🧠 Sixteen, thirty-nine, four — WHO 2021: 16 M/mL, 39 M total, 4% normal forms

• 🧠 Alcohol: AST is Twice ALT — AST:ALT > 2 in alcohol

Rapid-Revision Cards

• Casts come from

  Tamm-Horsfall protein, thick ascending limb → renal origin.

• RBC casts

  Glomerulonephritis.

• WBC casts

  Pyelonephritis.

• Muddy brown granular casts

  Acute tubular necrosis.

• Waxy casts

  Advanced CKD.

• Maltese cross

  Cholesterol in oval fat bodies → nephrotic.

• Haematuria

  > 3 RBC/HPF; dysmorphic = glomerular.

• Pyuria

  > 5 WBC/HPF.

• Blood on dipstick, no RBCs

  Haemoglobin or myoglobin.

• Renal glucose threshold

  ~180 mg/dL.

• Proteinuria limits

  Normal ≤ 150 mg/day; albumin 30–300 = moderately increased; > 3 g = nephrotic.

• KDIGO AKI

  Cr ↑ ≥ 0.3 in 48 h / ≥ 1.5× in 7 d / urine < 0.5 mL/kg/h × 6 h.

• Prerenal indices

  UNa < 10, FENa < 1%, BUN:Cr > 20.

• CKD evidence

  eGFR < 60 for ≥ 3 months; small kidneys.

• Light's criteria

  Protein ratio ≥ 0.5 / LDH ratio ≥ 0.6 / LDH ≥ 2/3 ULN.

• Pleural ADA

  > 40 U/L → TB pleuritis.

• SAAG ≥ 1.1

  Portal hypertension.

• SBP

  Ascitic neutrophils > 250/µL.

• Bacterial CSF

  Neutrophils, glucose ↓, protein > 100.

• GBS CSF

  Protein > 100 with few cells.

• WHO 2021 semen

  1.4 mL; 16 M/mL; 39 M total; motility 42%; progressive 30%; normal forms 4%.

• AST:ALT > 2

  Alcohol-related liver disease.

• Best thyroid screen

  TSH.

Sources

Facts in this article were checked against these documents.

• WHO laboratory manual for the examination and processing of human semen, sixth edition — World Health Organization, 2021 (link)

• MSD Manual Professional: Evaluation of the Renal Patient; Proteinuria; Hematuria; Nephrotic and Nephritic Syndrome; AKI; CKD — Merck & Co. (MSD Manuals), reviewed 2025–26 (link)

• MSD Manual Professional: Pleural Effusion; Ascites; Lumbar Puncture; Acute Bacterial Meningitis; Sperm Disorders — Merck & Co. (MSD Manuals), reviewed 2025–26 (link)

• MSD Manual Professional: Laboratory Tests of the Liver and Gallbladder; Overview of Thyroid Function; Hypothyroidism; Hyperthyroidism — Merck & Co. (MSD Manuals), reviewed 2025–26 (link)

• Competency Based Undergraduate Curriculum, Pathology (PA 22) — National Medical Commission, 2024

Test Yourself: Practice Questions

Twenty-nine questions in NEET PG style, most of them clinical vignettes.

Practice Questions (NEET PG pattern)

Original questions written in the style of the exam: +4 for a correct answer, −1 for a wrong one. Every answer was checked against the sources listed above. NEET PG papers are not published by NBEMS, so these are not past-paper questions.

Q1. A patient has haematuria, oedema and hypertension. Urine microscopy shows red-orange cylindrical casts containing red cells. These casts are virtually pathognomonic of

• (a) glomerulonephritis

• (b) nephrotic syndrome

• (c) pyelonephritis

• (d) acute tubular necrosis

Q2. A patient with acute kidney injury has many 'muddy brown' granular casts in the urine. The likely lesion is

• (a) prerenal azotaemia only

• (b) minimal change disease

• (c) acute tubular necrosis

• (d) cystitis

Q3. Broad, highly refractile casts formed in atrophic tubules are seen in

• (a) acute cystitis

• (b) advanced chronic kidney disease

• (c) healthy athletes

• (d) early nephritic syndrome

Q4. A child with heavy proteinuria and oedema has urine sediment showing oval fat bodies that form a Maltese cross under polarised light. The Maltese cross is due to

• (a) uric acid

• (b) calcium oxalate

• (c) cholesterol

• (d) haemosiderin

Q5. Nephrotic syndrome is defined by

• (a) > 3 g protein/day from a glomerular disorder plus oedema and low albumin

• (b) > 150 mg protein/day

• (c) RBC casts and hypertension

• (d) albumin 30–300 mg/day

Q6. In a patient with a urinary infection, finding WBC casts in the urine suggests

• (a) cystitis only

• (b) pyelonephritis (kidney involvement)

• (c) urethritis

• (d) contamination

Q7. Phase-contrast microscopy shows small red cells with spicules, folds and blebs. The bleeding is most likely from the

• (a) bladder

• (b) prostate

• (c) urethra

• (d) glomerulus

Q8. Microscopic haematuria is defined as

• (a) > 10 RBCs per HPF

• (b) > 5 WBCs per HPF

• (c) > 3 RBCs per high-power field

• (d) any RBC on dipstick

Q9. Red urine gives a uniform positive dipstick reaction for blood, but microscopy shows no red cells. The pigment is most likely

• (a) free haemoglobin or myoglobin

• (b) intact red cells

• (c) bilirubin

• (d) urobilinogen

Q10. With normal kidney function, glucose usually appears in the urine when blood glucose exceeds about

• (a) 180 mg/dL

• (b) 300 mg/dL

• (c) 100 mg/dL

• (d) 126 mg/dL

Q11. Persistent urinary albumin excretion of 30–300 mg/day is called

• (a) moderately increased albuminuria (formerly microalbuminuria)

• (b) nephrotic-range proteinuria

• (c) tubular proteinuria

• (d) normal

Q12. A hospitalised patient's creatinine rises from 1.0 to 1.4 mg/dL within 48 hours. By KDIGO this is

• (a) only significant if creatinine doubles

• (b) chronic kidney disease

• (c) acute kidney injury (rise ≥ 0.3 mg/dL in 48 h)

• (d) normal variation

Q13. Which set of urinary indices suggests prerenal AKI rather than tubular injury?

• (a) BUN:creatinine < 10

• (b) urine Na < 10, FENa < 1%, BUN:creatinine > 20

• (c) muddy brown casts

• (d) urine Na > 40, FENa > 1%

Q14. The most reliable evidence that kidney disease is chronic rather than acute is

• (a) eGFR < 60 mL/min/1.73 m² for ≥ 3 months

• (b) haematuria

• (c) a single high creatinine

• (d) normal-sized kidneys

Q15. Pleural fluid:serum protein ratio is 0.6 and LDH ratio is 0.7. The effusion is

• (a) an exudate (Light's criteria met)

• (b) a haemothorax

• (c) a transudate

• (d) a chylothorax

Q16. An exudative pleural effusion has adenosine deaminase of 60 U/L. The most likely diagnosis is

• (a) cirrhosis

• (b) tuberculous pleuritis

• (c) nephrotic syndrome

• (d) heart failure

Q17. Thoracentesis yields milky white fluid high in triglycerides. This is

• (a) chylothorax from thoracic duct damage

• (b) a transudate

• (c) empyema

• (d) haemothorax

Q18. A patient with ascites has serum albumin 3.0 g/dL and ascitic albumin 1.2 g/dL, with low ascitic protein. This points to

• (a) peritoneal tuberculosis

• (b) pancreatitis

• (c) portal hypertension (SAAG 1.8 ≥ 1.1)

• (d) peritoneal carcinomatosis

Q19. A patient with cirrhotic ascites has an ascitic neutrophil count of 400 cells/µL. This indicates

• (a) malignant ascites

• (b) spontaneous bacterial peritonitis

• (c) chylous ascites

• (d) normal cirrhotic ascites

Q20. CSF: raised pressure, thousands of cells mostly neutrophils, low glucose, protein 200 mg/dL. Diagnosis:

• (a) normal CSF

• (b) viral meningitis

• (c) Guillain-Barré syndrome

• (d) acute bacterial meningitis

Q21. CSF shows a few hundred lymphocytes with normal glucose. Most likely:

• (a) viral infection

• (b) tuberculous meningitis with low glucose

• (c) acute bacterial meningitis

• (d) subarachnoid haemorrhage

Q22. Subacute meningitis: CSF shows a few hundred lymphocytes, low glucose and raised protein. Likely causes include

• (a) tuberculosis or Cryptococcus

• (b) migraine

• (c) enterovirus

• (d) Guillain-Barré syndrome

Q23. CSF shows protein above 100 mg/dL with very few cells and normal glucose. Of these, the pattern fits

• (a) Guillain-Barré syndrome

• (b) brain abscess

• (c) bacterial meningitis

• (d) viral encephalitis

Q24. According to the WHO manual (2021), the 5th-centile sperm concentration of fertile men is

• (a) 39 million/mL

• (b) 16 million/mL

• (c) 20 million/mL

• (d) 60 million/mL

Q25. No spermatozoa are seen in replicate wet preparations of a semen sample. Before calling it azoospermia, WHO requires

• (a) a repeat after 1 week only

• (b) no sperm in the sediment of a centrifuged sample

• (c) a testicular biopsy

• (d) fructose testing only

Q26. A semen sample has pH 6.8 (no urine contamination). A pH below 7.2 suggests

• (a) prostatitis

• (b) lack of alkaline seminal-vesicle fluid

• (c) excess prostatic fluid

• (d) normal semen

Q27. A man with heavy drinking has AST 240 U/L and ALT 90 U/L. The AST:ALT pattern suggests

• (a) alcohol-related liver disease

• (b) obstructive jaundice

• (c) viral hepatitis

• (d) normal liver

Q28. A woman with minimal symptoms has raised TSH and normal free T4. This is

• (a) central hypothyroidism

• (b) T3 toxicosis

• (c) overt primary hypothyroidism

• (d) subclinical hypothyroidism

Q29. A thyrotoxic patient has undetectable TSH, normal T4 and raised T3. This is

• (a) euthyroid sick syndrome

• (b) subclinical hypothyroidism

• (c) TSH-secreting adenoma

• (d) T3 toxicosis

Active Recall

From memory: match each urinary cast to its disease, define haematuria and nephrotic-range proteinuria, give the KDIGO AKI criteria and prerenal indices, state Light's criteria and the SAAG and SBP cut-offs, compare CSF in bacterial, viral, TB meningitis and GBS, list the WHO 2021 semen values, and interpret AST:ALT and TSH patterns.