Arthocentesis correction for RBCs

Last updated on Sep 20, 2023 2 min read Clinical Calculators

This is a calculator used to correct the number of WBCs in an arthrocentesis for RBCs. The main option ("Conversion Factor") uses a heuristic for the WBC:RBC correction factor. This is not based on any research or studies, so take it with a grain of salt. To do a formal correction, use the panel called "The offical way" below

Synovial studies


Results
  • Measured WBC:
  • "Extra" WBC:
  • Corrected WBC:
  • Correction used:
The official way
Using serum studies

This uses the formula proposed by Elie et al. (J. Arthroplasty, 2008) to create the patient's individual "correction factor".

It calculates the ratio of WBCs to RBCs in the serum (the correction factor) and multiplies it by the RBCs seen in the arthrocentesis. This gets us the "extra" WBCs, which are subtracted from the observed WBCs.

Please note the units must be the same for both inputs. Many labs report serum RBC's in x 1012/L, whereas WBC's are x 109/L.

In the paper cited above the mean WBCserum & RBCsserum were 9.7 x 109/L & 4.05 x 1012/L in infected knees and 7.7 x 109/L & 4.35 x 1012/L in non-infected knees. Both of these correction factors are close to 500 RBCs per WBC (hence the heuristic).

Hunter Ratliff, MD, MPH
Hunter Ratliff, MD, MPH
Internal medicine resident

My research interests include epidemiology, social determinants of health, and reproducible research.