[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Clinical skills](https://durgical.com/en/clinical-skills/) › [Reading lab results](https://durgical.com/en/clinical-skills/areas/labs/) › Synovial fluid: septic, crystal or inflammatory?

# Synovial fluid: septic, crystal or inflammatory?

- Interpretation

Fluid drawn from a swollen joint is checked for cells, crystals and germs to tell infection from gout or other arthritis.

Updated Nov 13, 2026

## Key points

- In every acute hot joint, the first question is infection: aspirate before antibiotics when possible.
- Septic fluid is turbid with white cells usually over 50,000/µL and mostly neutrophils; a lower count does not exclude it.
- Gout: needle-shaped, strongly negatively birefringent urate crystals; pseudogout: rhomboid, weakly positively birefringent CPPD crystals.
- Crystals and infection can occur together; culture every hot-joint aspirate.
- Non-inflammatory fluid (osteoarthritis, trauma) has fewer than 2000 white cells/µL with few neutrophils.

On this page

- The basics Key points
- What it is
- Reference values and concepts
- Reading it step by step
- Patterns and common mistakes
- Next step
- Memory aid
- Elsewhere in DurgiPedia

- Clinical Synovial fluid groups (usual values; the groups overlap and ranges vary by laboratory).

- Study Test yourself

- Connections Connections

## What it is

Normal synovial fluid is a small amount of clear, pale, very viscous fluid, a filtrate of plasma enriched with hyaluronic acid made by the synovium, with very few cells. When a joint swells, aspiration and fluid analysis are the fastest way to the diagnosis, and the key question in an acute hot joint is always whether it is infected, because septic arthritis can destroy cartilage within days. The fluid is read in four parts: appearance and viscosity, the white cell count with the proportion of neutrophils, crystals under polarised light microscopy, and Gram stain with culture. Cell counts fall into broad groups: non-inflammatory (osteoarthritis, trauma), inflammatory (rheumatoid arthritis, crystal arthritis, reactive and other inflammatory arthritis) and septic, plus haemorrhagic fluid. The groups overlap, so a count alone never excludes infection. Urate crystals of gout are needle-shaped and strongly negatively birefringent; calcium pyrophosphate crystals of pseudogout are rhomboid or rod-shaped and weakly positively birefringent. Crystals and infection can coexist, so finding crystals does not end the search when the picture suggests sepsis.

## Reference values and concepts

- Normal (usual values, varying by laboratory): clear, pale yellow, high viscosity, white cells under about 200/µL, under 25% neutrophils.
- Non-inflammatory: clear yellow, high viscosity, about 200–2000 cells/µL, under 25% neutrophils.
- Inflammatory: cloudy yellow, low viscosity, about 2000–50,000 cells/µL, often over 50% neutrophils.
- Septic: turbid or purulent, usually over 50,000 cells/µL with over 75% neutrophils; Gram stain is often negative even when culture grows.
- Haemorrhagic: trauma, haemophilia or anticoagulation, fracture into the joint (fat droplets) and pigmented villonodular synovitis.
- Prosthetic joints become infected with much lower cell counts; involve orthopaedics early.
- Gonococcal arthritis often has negative joint cultures; test genital, rectal and throat sites as well.

## Reading it step by step

- Look at the fluid — clear suggests non-inflammatory; cloudy, inflammatory; purulent, septic; bloody, trauma or bleeding disorder.
- Check viscosity — inflammation breaks down hyaluronic acid, so the fluid becomes watery and does not string.
- Read the white cell count and neutrophil percentage — they place the fluid in a group; higher counts make infection more likely.
- Look for crystals under polarised light — needles negative for gout, rhomboids positive for pseudogout.
- Read the Gram stain and culture — they decide infection; a negative Gram stain does not exclude it.
- Put it together with the patient — fever, prosthesis, immunosuppression, injecting drug use or sexual risk raise suspicion of infection.
- If the picture is septic, treat as septic even when crystals are present — missing infection costs the joint.

## Synovial fluid groups (usual values; the groups overlap and ranges vary by laboratory).

| Group | Appearance | White cells/µL | Neutrophils | Crystals / culture | Typical causes |
| --- | --- | --- | --- | --- | --- |
| Normal | Clear, pale, viscous | Under about 200 | Under 25% | None / negative | Healthy joint |
| Non-inflammatory | Clear yellow, viscous | About 200–2000 | Under 25% | None / negative | Osteoarthritis, trauma |
| [Inflammatory](https://durgical.com/en/conditions/rheumatoid-arthritis/) | Cloudy yellow, watery | About 2000–50,000 | Often over 50% | Urate or CPPD in crystal disease / negative | Rheumatoid arthritis, gout, pseudogout, reactive arthritis |
| [Septic](https://durgical.com/en/conditions/septic-arthritis-of-the-knee/) | Turbid, purulent | Usually over 50,000 | Over 75% | May coexist / Gram stain or culture positive | Bacterial (often Staphylococcus aureus) or gonococcal arthritis |
| [Haemorrhagic](https://durgical.com/en/conditions/haemophilia-a-and-b/) | Bloody | Variable | Variable | Fat droplets in fracture | Trauma, haemophilia, anticoagulation |

## Patterns and common mistakes

- Treating a hot joint as gout because urate crystals were seen, and missing coexisting septic arthritis.
- Using a cell-count cut-off to exclude infection: septic joints can have counts under 50,000/µL, especially early or in immunosuppression.
- Starting antibiotics before aspiration, which makes the culture negative.
- Injecting a corticosteroid into a joint before infection has been excluded.
- Forgetting that the serum urate can be normal during an acute gout attack.
- Not testing other sites in a young sexually active adult with a hot joint and skin pustules or tenosynovitis (gonococcus).

## Next step

- Suspected septic arthritis: take blood cultures, start empirical intravenous antibiotics after the aspirate and call orthopaedics for joint washout or drainage.
- Crystal arthritis without signs of infection: treat the flare with an anti-inflammatory, colchicine or corticosteroid, and plan long-term urate control for gout.
- Inflammatory fluid without crystals or infection: check rheumatoid factor, anti-CCP and other autoimmune tests and refer to rheumatology.
- Haemorrhagic fluid: check coagulation and anticoagulant use and image for fracture.
- Call a senior at once for a prosthetic joint, a septic picture in an immunosuppressed patient, or sepsis.
- Review the culture at 48 hours and adjust treatment; follow the joint until swelling and function recover.

## Memory aid

Gout gives Negative Needles; Pseudogout gives Positive (rhomboid) crystals.

## Test yourself

Think of the answer, then tap the question to see it.

## A 70-year-old has a hot knee; fluid is cloudy with 25,000 cells/µL, 80% neutrophils, rhomboid weakly positive crystals and a negative Gram stain. Diagnosis?

Calcium pyrophosphate (pseudogout) arthritis is likely, but culture must be negative before infection is excluded, since crystals and sepsis can coexist.

## A man with diabetes has a hot knee and fever; the aspirate is purulent with 90,000 cells/µL, 95% neutrophils and urate crystals. What next?

Treat as septic arthritis with gout: blood cultures, empirical intravenous antibiotics after the aspirate and urgent orthopaedic review for washout. The crystals do not exclude infection.

## How are urate and calcium pyrophosphate crystals told apart under polarised light?

Urate crystals are needle-shaped and strongly negatively birefringent; calcium pyrophosphate crystals are rhomboid or rod-shaped and weakly positively birefringent.

## Knee fluid is clear and viscous with 800 cells/µL and 10% neutrophils in an older adult with chronic pain. Which group and likely cause?

Non-inflammatory fluid, most often osteoarthritis or mechanical damage. No crystals or infection are expected; manage the underlying joint disease.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/clinical-skills/synovial-fluid-septic-crystal-or-inflammatory/)

## Elsewhere in DurgiPedia

- [Knee effusion: patellar tap and bulge sign](https://durgical.com/en/clinical-exam/knee-effusion-patellar-tap-and-bulge-sign/)
- [Joint examination: inspection, palpation and range of motion](https://durgical.com/en/clinical-exam/joint-examination-inspection-palpation-and-range-of-motion/)
- [Synovial joints: capsule, synovial membrane and articular cartilage](https://durgical.com/en/histology/synovial-joints-capsule-synovial-membrane-and-articular-cartilage/)
- [Uric acid](https://durgical.com/en/biochemistry/molecules/uric-acid/)
- [Infectious arthritis](https://durgical.com/en/general-pathology/infectious-arthritis/)
- [Pseudogout (CPPD disease)](https://durgical.com/en/general-pathology/pseudogout/)

## Connections

## Related 4

- [Arthrocentesis of the knee](https://durgical.com/en/clinical-skills/arthrocentesis-of-the-knee/)
- [CRP, ESR and procalcitonin](https://durgical.com/en/clinical-skills/crp-esr-and-procalcitonin/)
- [Reading a full blood count](https://durgical.com/en/clinical-skills/reading-a-full-blood-count/)
- [Reading the CSF: bacterial, viral, fungal or bleeding?](https://durgical.com/en/clinical-skills/reading-the-csf-bacterial-viral-fungal-or-bleeding/)

## Related conditions 7

- [Septic Arthritis of the Knee](https://durgical.com/en/conditions/septic-arthritis-of-the-knee/)
- [Gout](https://durgical.com/en/conditions/gout/)
- [Gout of the Knee](https://durgical.com/en/conditions/gout-of-the-knee/)
- [Pseudogout (CPPD) of the Knee](https://durgical.com/en/conditions/pseudogout-of-the-knee/)
- [Rheumatoid arthritis](https://durgical.com/en/conditions/rheumatoid-arthritis/)
- [Reactive arthritis](https://durgical.com/en/conditions/reactive-arthritis/)
- [Haemophilia A and B](https://durgical.com/en/conditions/haemophilia-a-and-b/)

## Microorganisms 2

- [Staphylococcus aureus](https://durgical.com/en/microbiology/staphylococcus-aureus/)
- [Neisseria gonorrhoeae](https://durgical.com/en/microbiology/neisseria-gonorrhoeae/)

## In Pharmacology 5

- [Colchicine](https://durgical.com/en/pharmacology/colchicine/)
- [Nonsteroidal anti-inflammatory drugs (NSAIDs)](https://durgical.com/en/pharmacology/classes/nonsteroidal-anti-inflammatory-drugs/)
- [Corticosteroids (glucocorticoids)](https://durgical.com/en/pharmacology/classes/corticosteroids/)
- [Vancomycin](https://durgical.com/en/pharmacology/vancomycin/)
- [Ceftriaxone](https://durgical.com/en/pharmacology/ceftriaxone/)

[← All clinical skills](https://durgical.com/en/clinical-skills/)

Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: [Synovial Fluid Analysis](https://www.ncbi.nlm.nih.gov/books/NBK537114/) (StatPearls (NCBI Bookshelf), 2023).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/clinical-skills/synovial-fluid-septic-crystal-or-inflammatory/