A 60 year old male presents to ED with a 3 day history of a swollen, red painful left ankle.
He has felt systemically unwell and is unable to weight bear. He returned from Thailand 1 week previously. There is no history of trauma. An ankle athrocentesis is performed and below are the initial results:
Description – 30ml of turbid fluid
Microscopy – Leucocytes 45667 /uL
- Polymorphs 95%
- Mononuclear 5%
- Erthrocytes 50 /uL
- No crystals observed
Microscopy – No bacteria seen
- Describe and interpret the above results, including a differential diagnosis
- Describe the process of aspirating an ankle joint
Answer
The above ankle aspirate shows turbid fluid with a markedly raised white cell count, with a predominate neutrophilia. There is a small amount of RBC, with no crystal or bacteria seen. In this clinical context, the most likely diagnosis would be septic arthritis, but other differentials diagnosis need to be considered. See bleow:
| Cause | Fluid | WCC | PMN | Crystals | Bacteria |
| Noninflammatory | |||||
| OA or Traumatic Arthritis | Straw coloured, clear | <3000 | <25% | Nil | Nil |
| Inflammatory | |||||
| RA | Straw coloured, cloudy | 3000-20000 | <75% | Nil | Nil |
| Gout | Straw coloured, cloudy | 3000-20000 | <75% | Negative Birefringence | Nil |
| Pseudogout | Straw coloured, cloudy | 3000-20000 | <75% | Positive Birefringence | Nil |
| Infective | Yellow and turbid | >50 000 (92% Specificity)
(If >25000 has a 70% specificity |
>90% | Nil | <25% positive in gonococcal infection
50-75% positive in non gonococcal infection |
Ankle Aspiration:
Indication – to investigate cause of ankle effusion with no history of trauma. Usually to diagnose septic arthritis
Contraindications – overlying cellulitis, coagulopathy (relative in patients taking oral anticoagulants)
Process – identify the following landmarks – tibialis anterior, extensor hallucis longus, extensor digitorum longus, medial and lateral malleoli. Areas for aspiration – between medial malleolus and TA, or between TA and EHL, or between lateral malleolus and EDL. Use sterile technique, and LA. Use a 18 to 20G needle and 20ml syringe. Remove as much flid as possible. Use EDTA blood tube for cell count, and urine specimen jar for MCS. Send to lab for WCC, MCS and crystals.
Complications – infection, bleeding, pain, damage to bone and cartilage.