PRINCIPLE:

A preparation of clinical material is used to facilitate the observation of uric acid crystals and/or other crystals.

MATERIAL NEEDED:

• Clean glass slide and cover slip.
• Polarized light
• Red compensator (if available).

PROCEDURE:

1. Place a drop of synovial fluid on a clean glass slide. Apply cover slip to make a thin mount.
2. The wet prep should be examined immediately to demonstrate intracellular crystals.
3. Scan microscopically under low power with reduced light looking for cells. When cells found, switch to polarized light. Adjust the light flow accordingly. Look for intracellular and extracellular crystals.
4. Identification of urate and/or calcium pyrophosphate dehydrate requires the addition of a first class-order red compensator. Insert the compensator between the polarizer and the analyzer.

INTERPRETATION:

The types of crystals that may be seen in synovial fluid include: monosodium urate, calcium pyrophosphate dehydrate, cholesterol, steroid, and apatite.

 The presence of monosodium urate crystals is usually diagnostic of gout. The crystals may be intracellular, extracellular, or both. They are needle-like with pointed ends and measure 1-30 μl in length. The presence of intracellular crystals indicates the acute stage of gout.

 Monosodium urate crystals grow in elongated prisms that have a negative optical sign of birefringence, which generates a yellow (subtraction) interference color when the long axis of the crystal is oriented parallel to the slow axis of the first order retardation plate (Figure 6(a)). Rotating the crystals through 90 degrees changes the interference color to blue (addition color; Figure 6(b)).

Calcium pyrophosphate dehydrate are rod-like, plate-like, or rhomboid. They are weakly birefringent of polarized light and appear pale against the dark background. They give a blue sign of birefringence with a red compensator.

Calcium pyrophosphate dehydrate (CPPD) crystals is associated with pseudo-gout and chondrocalcinosis. These crystals are most commonly seen in synovial fluid samples from patients with degenerative arthritis but may also be seen in hereditary forms of pseudogout, in arthritides associated with metabolic diseases, such as hyper-parathyroidism, hypothyroidism, diabetes mellitus, and hemochromatosis, and occasionally together with monosodium urate crystals in gout.

In contrast to Monosodium urate crystals, pseudo-gout pyrophosphate crystals, which have similar elongated growth characteristics, exhibit a blue interference color (Figure 6(c)) when oriented parallel to the slow axis of the retardation plate and a yellow color (Figure 6(d)) when perpendicular. The sign of birefringence can be employed to differentiate between gout crystals and those consisting of pyrophosphate.

 Cholesterol crystals appear as strongly birefringent plates, often with notched margins. They may occasionally have the appearance of needles or rhomboids.

 Other birefringent materials:

Fragments of cartilage may also appear birefringent under polarized light but have irregular margins.

Dirt from unclean glassware, and talcum crystals, talcum crystal has a Maltese-cross appearance.

QUALITY CONTROL:

Examine slide or printed material of wet preparation to identify crystals or recognize absence of crystals. Slide or printed material can be provided by St. Peter’s Hospital.

 PRECAUTION:

Powdered EDTA, oxalate, or lithium heparin should not be used as anticoagulant, since each may associate with crystal formation. Calcium pyrophosphate crystals, which are weakly birefringent may be missed unless the polarizing equipment is adequate.

 Under polarized light, corticosteroids may appear identical to monosodium urate or calcium pyrophosphate crystals. Therefore, it is essential to know whether a previous intra-articular injection was given. Such crystals may be present in synovial fluid for a month or longer following the injection.

 REFERENCES:

 Body Fluids Laboratory Examination of Amniotic, Cerebrospinal, seminal, Serous & Synovial Fluids  Carl R. Kjeldsberg, Joseph A. Knight,  Second edition. American Society of clinical Pathologist  Press, Chicago

 Slides from European League against Rheumatism (with permission).

 Identification of Gout Crystals, OLYMPUS Microscopy Resource Center.

 PLACED INTO SERVICE: November 1 2005

 SUPERSEDES PREVIOUS PRACTISES.