Synovial fluid

7 ASCP MLS practice questions on Synovial fluid, part of Urinalysis and Body Fluids. The first 7 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Hard

   An 82-year-old woman has a large, painful right shoulder effusion that has built up over months. X-ray shows destruction of the joint and loss of the rotator cuff. Aspirate: 55 mL, blood-tinged; WBC 650/µL [0.65 x 10^9/L], 75% mononuclear cells; glucose close to the blood level; Gram stain negative. Compensated polarized light shows no birefringent crystals, and no rods or rhomboids are seen inside or outside cells. The wet preparation contains small, shiny, nonbirefringent clumps that stain bright orange-red with alizarin red S. Which crystal is most likely present?

   1. ABasic calcium phosphate (apatite)
   2. BMonosodium urate monohydrate
   3. CCalcium pyrophosphate dihydrate
   4. DCalcium oxalate monohydrate

   Show answerHide answer

   Answer: A · Basic calcium phosphate (apatite)

   The fluid is noninflammatory (WBC below 2,000/µL, mostly mononuclear cells, normal glucose), so infection and an acute crystal flare are unlikely. Basic calcium phosphate crystals, such as hydroxyapatite, are too small to see one by one; they form shiny, nonbirefringent clumps that stain with alizarin red S, a calcium stain. A destructive shoulder arthropathy with a large, blood-tinged, low-count effusion in an elderly woman is the classic Milwaukee shoulder picture. Calcium pyrophosphate forms visible rods and rhomboids with weak positive birefringence, and urate forms strongly negatively birefringent needles. Definitive identification of apatite needs electron microscopy or other special methods.

   • B: Urate forms needles with strong negative birefringence, and none are seen.
   • C: Calcium pyrophosphate forms visible rods and rhomboids with weak positive birefringence, and none are seen.
   • D: Calcium oxalate, seen mainly in dialysis patients, forms birefringent rods or bipyramids.

2. Question 2 · Hard

   Synovial fluid contains needle-shaped crystals that are strongly negatively birefringent: yellow when aligned parallel to the compensator's slow axis. What are they?

   1. ACorticosteroid crystals
   2. BCholesterol crystals
   3. CCalcium pyrophosphate (pseudogout)
   4. DMonosodium urate (gout)

   Show answerHide answer

   Answer: D · Monosodium urate (gout)

   Monosodium urate crystals are needles that are bright and strongly negatively birefringent, yellow when parallel to the slow axis of the red compensator. They confirm gout. Calcium pyrophosphate crystals are rhomboid, weakly positively birefringent, and blue when parallel. Crystals inside white cells show an active inflammatory process.

   • A: Injected steroid crystals vary in shape and are not typically strongly negative needles.
   • B: Cholesterol crystals are flat plates with notched corners.
   • C: Calcium pyrophosphate crystals are rhomboid and weakly positively birefringent (blue when parallel).

3. Question 3 · Hard

   Synovial fluid is diluted 1:20 in hypotonic saline and loaded on a Neubauer hemocytometer. A total of 180 white cells are counted in 4 large squares (1 mm² each, depth 0.1 mm). The WBC count is:

   1. A900/µL (0.9 x 10^9/L)
   2. B36,000/µL (36 x 10^9/L)
   3. C9,000/µL (9.0 x 10^9/L)
   4. D450/µL (0.45 x 10^9/L)

   Show answerHide answer

   Answer: C · 9,000/µL (9.0 x 10^9/L)

   Cells/µL = cells counted x dilution ÷ (area counted x depth) = 180 x 20 ÷ (4 mm² x 0.1 mm) = 3,600 ÷ 0.4 = 9,000/µL (9.0 x 10^9/L). Hypotonic saline lyses red cells without forming the mucin clot that acetic acid causes. A count of this size with mostly neutrophils fits an inflammatory fluid. Very viscous fluid can be pretreated with hyaluronidase so that it fills the chamber evenly.

   • A: 900/µL leaves out the 0.1 mm depth; each large square holds 0.1 µL, not 1 µL.
   • B: 36,000/µL leaves out dividing by the 4 squares counted.
   • D: 450/µL leaves out the 1:20 dilution.

4. Question 4 · Medium

   Knee fluid is clear, yellow and highly viscous; WBC 650/µL (650 x 10^6/L), 18% neutrophils, no crystals, and glucose close to the blood level. It is best classified as:

   1. ANoninflammatory, as in osteoarthritis
   2. BInflammatory, as in rheumatoid arthritis
   3. CCrystal-induced, as in gout
   4. DSeptic, as in bacterial arthritis

   Show answerHide answer

   Answer: A · Noninflammatory, as in osteoarthritis

   A WBC below about 1,000 to 2,000/µL (cutoffs vary by source) with fewer than 30% neutrophils, good viscosity and a near-normal glucose defines a noninflammatory fluid, typical of osteoarthritis. Normal synovial fluid has fewer than about 200 WBC/µL. Inflammatory, crystal-induced and septic fluids have higher counts with mostly neutrophils, lower viscosity and often a lower glucose. Classifying the fluid narrows the causes while culture and crystal results are pending.

   • B: Inflammatory fluids have a WBC above about 2,000/µL with mostly neutrophils.
   • C: Gout fluid is inflammatory and contains urate crystals.
   • D: Septic fluids usually have tens of thousands of WBCs, mostly neutrophils, and a low glucose.

5. Question 5 · Hard

   A 45-year-old man has fever and a hot, swollen knee. Synovial fluid is cloudy with low viscosity; WBC 110,000/µL (110 x 10^9/L), 93% neutrophils; glucose 22 mg/dL (1.2 mmol/L) with blood glucose 96 mg/dL (5.3 mmol/L); no crystals are seen. What is the most likely diagnosis?

   1. AAcute gout
   2. BOsteoarthritis
   3. CRheumatoid arthritis
   4. DSeptic arthritis

   Show answerHide answer

   Answer: D · Septic arthritis

   A WBC around 100,000/µL with more than 90% neutrophils, low viscosity and a synovial glucose far below the blood level (here 74 mg/dL [4.1 mmol/L] lower) is the septic pattern, because bacteria and neutrophils consume glucose. Gram stain and culture are urgent, and the joint is drained, since cartilage can be destroyed within days. Gout can also give a very high count, so a careful crystal search is still made. Fever, no crystals and the very low glucose favor infection.

   • A: Acute gout shows urate crystals, and the glucose usually falls less.
   • B: Osteoarthritis fluid is noninflammatory, with fewer than about 2,000 WBC/µL.
   • C: Rheumatoid fluid is inflammatory, but counts are usually much lower than 100,000/µL.

6. Question 6 · Easy

   In compensated polarized light microscopy of synovial fluid, the red compensator is used to:

   1. AMake birefringent crystals shine on a dark background
   2. BStain crystals so they can be seen in bright field
   3. CLyse the red cells that hide crystals in the fluid
   4. DShow whether a crystal's birefringence is positive or negative

   Answer and explanation in LearnFree account

7. Question 7 · Medium

   Why is synovial fluid not diluted with the usual acetic acid WBC diluent for a manual cell count?

   1. AIt lyses neutrophils along with red cells
   2. BIt precipitates hyaluronate into a mucin clot
   3. CIt is too weak to lyse synovial red cells
   4. DIt dissolves monosodium urate crystals

   Answer and explanation in LearnFree account