Gout crystals - what are they like; how are they identified ?

This page about gout crystals - and testing for their presence -  was last reviewed or updated on 4 August 2017  

SUMMARY This article should help people who are trying to find out if they have gout, or something else, or something similar.


The joint aspiration (aka arthrocentesis) test is the best test for gout, much better than measuring the blood uric acid of someone who is suspected of having gout. But should you find out you have a high blood uric acid level - above 6.5 mg/dL women and 7.0 mg/dL men - that is not proof of gout, merely indicative.

Although a higher blood uric acid level is more likely to mean gout, the joint aspiration (arthrocentesis) test is proof of gout if it's done correctly.In truth, it is the only real test that can prove you have  gout. But this test isn’t faultless, and may not work, at least on the first occasion. There are some snags associated with it, and you might run into them.  Despite the snags, it's a test you really should have done as soon as you suspect you have gout or arthritis.

In theory, what happens here is that a sample of synovial fluid (the fluid stored in a cavity in the gouty joints -see the photo below) is taken from the gout attack joint. It is then viewed  under an illuminated, polarised, (polarized) microscope. This is a microscope using

both a polarising (polarizing) and a compensating filter. The polarising filter is  somewhat similar to the polarising filter you may use on your camera lens. It reveals whether there are MSU gout crystals or not in the synovial fluid, to the skilled eye.

Gout's crystals are negatively birefringent (aka birifringent). Under a polarising (polarizing) filter, they are yellow or blue in colour (color), depending on their position in relation to the polarising and compensator filters.You won't see them with your naked eye. If correctly identified, these crystals confirm you have gout.

Antoni van Leeuwenhoek, the first human to see them under a microscope he had invented in the 1660's, said the width of 1,000 needle shaped uric acid crystals lying alongside each other would not equal the width of a human hair. He said they can be up to 1/8" long, which is long for something that fits inside a body joint. 

Take a look at Gout's crystals - the photo here with a maroon background is of needle shaped gout crystals  under polarised (polarized) light.                                                                                                                         

                                                                                                                                                         Gout crystals photo : Bob J Galindo

If your crystals are correctly identified as gout-causers by the microscopist - he/she needs to be experienced at this - then you have gout. But identifying them is not as easy as it's often described.

Needles in a haystack ? No, but here are the snags: 

Firstly, your habitual PCP/GP doctor probably won’t have the necessary polarising (polarizing) microscope, nor will every rheumatologist or another specialist you see. But your nearest hospital should have one. So to get this test done, there may have to be a search to locate the nearest one, and operated by  the all-important trained microscopist.

Secondly, the sample of synovial fluid taken from your joint may not hold enough uric acid crystals - remember the crystals have formed, probably haphazardly, from uric acid in the blood. They have to be large enough for an examiner to be certain of what they are. They are not all as long as Van Leeuwenoek noted they could be. 

Thirdly, the person managing your test may not be able to collect enough synovial fluid. Some joints don’t have much – e.g the big toe joints. If you have knee gout, there should be plenty of synovial fluid.

Finally, you might note this. One of our readers advised us that an aspiration on an ankle joint did not go well, because the crystals had started to dissolve and were nor easily seen. So he advised to get a joint aspiration (arthrocentesis) done as soon as you can, i.e before the crystals dissolve and the attack ends.

So is it gout ? 

You and you doctor may have to decide if you  have gout or pseudo gout. Synovial fluid for a pseudo gout inspection  is also aspirated  from the joint. It is then put under the lens and eye of the microscope and its operator. The microscopist must be able to tell the difference between gout and pseudo gout crystals. The crystals of pseudo gout CPPD - Calcium Pyrophosphate Deposition Disease crystals (or for short  Calcium Pyrophosphate Crystals), are weakly but positively birefringent (birifringent) and are also coloured yellow and blue.

So what's the crystals' difference ? 

Gout crystals are needle shaped but the CPPD crystals of pseudo gout are rhomboid or square shaped. This is something a trained operator will note. Their colours (colors) are similar but different. Both are coloured (colored) blue or yellow, depending on their position relative to the compensating filter. But while gout crystals are blue in one position, pseudo gout crystals will be yellow in that position. The opposite colour incidence is the case for the other position.

This first military tank, a British one from World War 1, is rhomboid shaped.  Pseudo gout crystals also may be square shaped.


To sum it up .You have gout if the microscope operator correctly identifies the needle shaped gout crystals. They are long (around 1/8 inch), very thin, flared along the horizontal sides, blue or yellow in colour (color), with sharp pointed ends, and importantly crystals with negative birefringence. 

That might save you two years or more trying to decide what you have - gout, pseudo gout, rheumatoid arthritis, bacterial arthritis, all of which the joint aspiration test helps to identify.  

Related page

Visit our main page about gout and pseudo gout - how do they differ ?

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