What is the role of lab tests in the workup of prepatellar bursitis (housemaid's knee)?

Updated: Sep 15, 2020
  • Author: Divakara Kedlaya, MBBS; Chief Editor: Dean H Hommer, MD  more...
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Laboratory studies are not usually indicated to diagnose prepatellar bursitis. However, analysis of fluid aspirated from the bursa is important in differentiating septic bursitis (SB) from nonseptic bursitis (NSB). The fluid should be sent for analysis, since the prepatellar bursa is commonly a site of infection. (See the table below.)

Table 1. Characteristics of bursal fluid in patients with septic and nonseptic prepatellar bursitis (Open Table in a new window)


Septic bursitis (SB)

Nonseptic bursitis (NSB)



Serosanguineous, straw colored, or bloody

White blood cell count per µL

1500-300,000; mean 75,000

50-10,000; usually < 3000

Differential count

Polymorphonuclear (PMN) cells

Predominantly mononuclear cells

Bursal fluid–to–serum glucose ratio

< 50%


Gram stain

Positive in 70%





Evaluate the aspirated fluid for WBC count, protein, lactate, glucose, crystals, and Gram stain results. Typical findings in septic bursitis include the following:

  • WBC count of 50,000/µL or greater

  • Elevated protein

  • Elevated lactate

  • Decreased glucose

  • Gram stain results specific to bacteria

In crystal-induced diseases causing bursitis, findings include monosodium urate crystals (gout), calcium pyrophosphate crystals (pseudogout), and cholesterol crystals (rheumatoid arthritis).

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