Expansion
Appearance, cell count, crystals and Gram stain
Expansion
| Normal | Non-inflammatory | Inflammatory | Septic | |
|---|---|---|---|---|
| Appearance | Clear, viscous | Clear, yellow | Cloudy, yellow | Purulent |
| White cells per mm3 | Under 200 | Under 2000 | 2000 to 50,000 | Over 50,000 |
| Neutrophils | Under 25 per cent | Under 25 per cent | Over 50 per cent | Over 75 per cent |
| Examples | Osteoarthritis, trauma | Gout, rheumatoid, reactive | Bacterial infection |
The thresholds are a guide, not a rule: septic arthritis occurs with counts below 50,000, particularly in prosthetic joints, in the immunosuppressed and in partially treated infection.
Crystals under polarised light
| Gout | Pseudogout | |
|---|---|---|
| Crystal | Monosodium urate | Calcium pyrophosphate |
| Shape | Needle shaped | Rhomboid |
| Birefringence | Negative | Positive |
| Colour parallel to the compensator | Yellow | Blue |
The mnemonic is that gout crystals are needle shaped and negatively birefringent, and both words carry an “n”.
Septic arthritis
- A hot, swollen, tender joint with restricted movement is septic until proven otherwise
- Aspirate before antibiotics wherever possible
- Gram stain is positive in only about half, and culture in about 70 per cent
- Staphylococcus aureus is commonest overall; think gonococcus in a sexually active young adult with migratory arthritis, tenosynovitis and a rash
- A normal white cell count, C reactive protein or temperature does not exclude it
- Prosthetic joints must be aspirated in theatre, not on the ward