Interpret · Fluids & specimens
Every tapped space asks the same questions: cells, chemistry, organisms, and a comparison against serum. CSF, pleural, ascitic, and synovial fluid, with the criteria (Light's, SAAG) applied on real numbers.
Cells, protein, glucose, and the patterns of meningitis.
A 19-year-old student with fever, headache, and photophobia. LP performed after a normal neurologic exam.
Fluid analysis
| Opening pressure | 32 cm H2O | 10–20 |
|---|---|---|
| Appearance | Turbid | |
| White cells | 2400 ×10⁶/L | < 5 |
| Differential | 92% neutrophils | |
| Protein | 2.4 g/L | 0.15–0.45 |
| CSF glucose | 1.1 mmol/L | |
| Gram stain | Gram-negative diplococci |
Paired serum
| Serum glucose | 6.2 mmol/L |
|---|
Which single pairing here is most characteristic of bacterial meningitis?
A 24-year-old with three days of headache and fever, well-appearing.
Fluid analysis
| Opening pressure | 18 cm H2O | 10–20 |
|---|---|---|
| Appearance | Clear | |
| White cells | 180 ×10⁶/L | < 5 |
| Differential | 88% lymphocytes | |
| Protein | 0.68 g/L | 0.15–0.45 |
| CSF glucose | 3.6 mmol/L | |
| Gram stain | No organisms |
Paired serum
| Serum glucose | 5.8 mmol/L |
|---|
How does this fluid differ from the bacterial pattern?
A 41-year-old with a thunderclap headache eleven hours ago, now improving. CT head at hour ten: normal. The lumbar puncture is done at hour twelve.
Fluid analysis
| Opening pressure | 18 cm H2O | 10 to 20 |
|---|---|---|
| Appearance | Faintly yellow supernatant after centrifugation | |
| Red cells, tube 1 | 48,000 ×10⁶/L | |
| Red cells, tube 4 | 44,000 ×10⁶/L | |
| White cells | 8 ×10⁶/L | < 5 |
| Spectrophotometry | Bilirubin peak present |
Which finding distinguishes subarachnoid blood from a traumatic tap?
Light's criteria and the exudate question.
A 57-year-old, one week of cough and fever, with a moderate right effusion. Thoracentesis is performed.
Fluid analysis
| Appearance | Cloudy yellow |
|---|---|
| Pleural protein | 38 g/L |
| Pleural LDH | 580 U/L |
| Pleural pH | 7.12 |
| Pleural glucose | 2.1 mmol/L |
| White cells | 18 000 ×10⁶/L, neutrophilic |
Paired serum
| Serum protein | 62 g/L |
|---|---|
| Serum LDH | 210 U/L (upper normal 220) |
Apply Light's criteria. What kind of effusion is this, and what is the added worry?
A 78-year-old admitted with decompensated heart failure. The larger right effusion is tapped for comfort and analyzed.
Fluid analysis
| Appearance | Clear, pale yellow |
|---|---|
| Fluid protein | 18 g/L |
| Fluid LDH | 88 U/L |
| Cell count | 220 ×10⁶/L, lymphocyte predominant |
| pH | 7.44 |
| Glucose | 5.8 mmol/L |
Paired serum
| Serum protein | 62 g/L |
|---|---|
| Serum LDH | 180 U/L |
Transudate or exudate, and what follows from the answer?
Day 5 of treated pneumonia: fevers return, and the effusion has grown. The tap yields turbid fluid.
Fluid analysis
| Appearance | Turbid, foul-smelling | |
|---|---|---|
| pH | 7.08 | > 7.30 |
| Glucose | 1.6 mmol/L | ≈ serum |
| LDH | 2,400 U/L | |
| Protein | 42 g/L | |
| Gram stain | Gram-positive cocci in chains |
Paired serum
| Serum protein | 62 g/L |
|---|---|
| Serum LDH | 210 U/L |
Fluid pH 7.08, glucose 1.6, organisms on Gram stain. What does this fluid demand?
SAAG, neutrophil counts, and SBP.
A 55-year-old with known cirrhosis and new abdominal distension. Diagnostic paracentesis.
Fluid analysis
| Appearance | Straw-colored |
|---|---|
| Ascitic albumin | 8 g/L |
| Ascitic total protein | 18 g/L |
| Neutrophils (PMN) | 120 ×10⁶/L |
Paired serum
| Serum albumin | 26 g/L |
|---|
SAAG = 26 − 8 = 18 g/L. What does that number classify?
A 55-year-old with known cirrhosis and ascites, admitted with low-grade fever, diffuse abdominal tenderness, and a creatinine creeping up.
Fluid analysis
| Appearance | Slightly cloudy |
|---|---|
| Cell count | 1,180 ×10⁶/L |
| Neutrophils (PMN) | 640 ×10⁶/L |
| Fluid protein | 12 g/L |
| Fluid albumin | 6 g/L |
| Culture | pending |
Paired serum
| Serum albumin | 26 g/L |
|---|
The culture is still pending. What does the cell count already establish?
A 61-year-old woman with three months of abdominal distension and weight loss. No liver history. The ascitic tap is analyzed with paired serum.
Fluid analysis
| Appearance | Straw-colored |
|---|---|
| Albumin | 24 g/L |
| Protein | 38 g/L |
| White cells | 380 ×10⁶/L, lymphocyte predominant |
| Cytology | Sent, pending |
Paired serum
| Serum albumin | 32 g/L |
|---|
SAAG = 32 − 24 = 8 g/L. What does a low SAAG (below 11 g/L) tell you?
Cell counts, crystals, and the hot joint.
A 61-year-old with diabetes: one day of a swollen, exquisitely painful right knee and a temperature of 38.4.
Fluid analysis
| Appearance | Purulent, low viscosity | |
|---|---|---|
| White cells | 82 000 ×10⁶/L | < 200 |
| Differential | 94% neutrophils | |
| Crystals | None seen | |
| Gram stain | Gram-positive cocci in clusters |
Had the crystal exam shown urate crystals, would that have excluded septic arthritis?
A 48-year-old with recurrent abrupt first-MTP attacks. An acute knee effusion is tapped during an attack.
Fluid analysis
| Appearance | Cloudy yellow | |
|---|---|---|
| White cells | 24 000 ×10⁶/L | < 200 |
| Differential | 80% neutrophils | |
| Crystals | Needle-shaped, strongly negatively birefringent, intracellular | |
| Gram stain | No organisms |
Which crystal description fits gout?
An 84-year-old admitted with pneumonia develops an acutely hot, swollen knee on day 3. Radiograph: cartilage calcification along the menisci.
Fluid analysis
| Appearance | Cloudy yellow |
|---|---|
| White cells | 19,000/µL, neutrophil predominant |
| Crystals | Rhomboid, weakly POSITIVELY birefringent, some intracellular |
| Gram stain | No organisms |
| Culture | Pending |
Rhomboid, positively birefringent crystals. The diagnosis and its favorite context?