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AMBULATORY PRACTICE
How to Obtain Useful Information From a Grossly
Blood-Contaminated Synovial Fluid Sample
James L. Carmalt, MA, VetMB, MVetSc, FRCVS, Diplomate ABVP (Eq), ACVS*;
Imma Roquet, DVM, MVetSc; and Steve Hendrick, DVM, DVSc
Authors’ address: Department of Large Animal Clinical Sciences, Western College of Veterinary
Medicine, 52 Campus Drive, University of Saskatchewan, Saskatoon, SK S7N 5B4 Canada; e-mail:
[email protected]. *Corresponding and presenting author. © 2013 AAEP.
1.
Introduction
Synovial fluid examination in the horse is a routine
procedure in equine practice. One of the most valuable uses of synovial fluid (SF) analysis is to determine the presence or absence of infection within a
synovial structure.1–3
Normal equine SF is clear to straw-colored and
highly viscous. It is an ultrafiltrate of plasma,
therefore similar concentrations of glucose and electrolytes are expected.3 The total protein (TP) concentration and white blood cell count (WBC),
however, are much lower, and the prevailing cell
type is mononuclear. SF should contain very few to
no erythrocytes. The reported normal values of SF
WBC range from ⬍300 to 1000 cells per microliter,
containing mainly mononuclear cells with ⬍10%
neutrophils on cytologic examination.3–7 Normal
TP concentrations have been variably described as
⬍25% to 35% of the systemic plasma TP and ⬍0.8 to
2.5 g/dL.3,4
Values of WBC and TP above 10 ⫻ 109/L and 50
g/L, respectively, with ⬎80% neutrophils, are highly
suggestive of septic arthritis; however, lower levels
do not necessarily rule this out.3 The definitive
diagnosis of a septic synovial structure of bacterial
NOTES
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2013 Ⲑ Vol. 59 Ⲑ AAEP PROCEEDINGS
origin is a positive culture of the SF and/or synovial
membrane.5
Many instances occur in equine practice, in which
clients are faced with the difficult decision of
whether or not to initiate aggressive treatment because of economic constraints. Unfortunately, it is
not uncommon for an equine practitioner to be unable to provide a definitive diagnosis of septic arthritis for multiple reasons, including an iatrogenically
blood contaminated sample. This report shows
how to obtain diagnostically useful synovial fluid TP
and WBC values from a blood-contaminated sample,
given a concurrent complete blood count.
2.
Materials and Methods
To be able to compute useful WBC counts and TP
values from a blood contaminated synovial sample,
one must obtain both a synovial fluid sample and a
peripheral venous blood sample (usually by means
of the jugular vein).
The hematocrit (HCT) and WBC counts of both
the synovial fluid and blood sample are obtained, as
well as the TP concentration of the synovial fluid in
the laboratory.
AMBULATORY PRACTICE
Table 1. Formula for Calculation of Uncontaminated Total Protein From
a Blood-Contaminated Synovial Fluid Sample
Laboratory Values
Horse Name/Number
9
3.49
2.90
9.92
7.00
11.10
30.50
Synovial WBC, *10 /L
Synovial TP, g/L
Blood WBC, *109/L
Blood TP, g/L
Synovial hematocrit, %
Blood hematocrit, %
Calculated values
Uncontaminated TP, g/L
Uncontaminated WBC,
*109/L
1.25
0.78
A ratio of HCT must be calculated to determine
the degree of contamination for the synovial fluid
sample.
Ratio HCT ⫽ HCT SF /HCT Blood
(1)
The difference between the values obtained from
the contaminated synovial fluid sample and those
that would have been obtained in a completely
blood-free sample is then calculated as:
TP DIFF ⫽ ⫺0.063⫺0.0542 (HCT SF )
⫹ 6.34(RatioHCT)
(2)
WBC DIFF ⫽ ⫺0.01916 ⫹ 0.01373 WBC Blood
⫹ 0.08689 HCT SF ⫹ 0.01479(WBC Blood * HCT SF )
(3)
As TPDIFF ⫽ TP (contaminated SF sample)⫺TP
(theoretical blood-free sample), simple substitution
is used to calculate the blood-free value for this
variable. Similarly, WBCDIFF ⫽ WBC (contaminated SF sample)⫺WBC (theoretical blood-free sample) can be used to calculate the blood-free synovial
fluid WBC count.
This process seems cumbersome; however, in the
authors’ practice, a Microsoft Excel spreadsheet has
been constructed such that entering the laboratory
values will automatically generate blood-free– calculated values without having to resort to using a
calculator every time (Tables 1 and 2).
3.
Discussion
The mathematical models used above were generated in a two-part study.8 Briefly, peripheral venous blood and SF was obtained from 10 adult
horses. The SF samples were split (into six) and
subsequently contaminated with the use of autologous blood added in 10% increments from 0% to
50%. In some horses, autologous plasma was also
added to artificially raise the baseline total protein
concentration. An automated cell count was obtained followed by direct smears in both the SF and
venous blood samples. The packed cell volume
(PCV) and the TP concentration were measured with
a hand-held refractometer after sample centrifugation. Dummy variables (RatioHCT, WBCDIFF, TPDIFF, and NPDIFF) were created as the difference
between the variable count of a sample with a
known percentage of contamination and one with
0% contamination. With the use of a combination
of best-subset regression and linear regression techniques, mathematical models were generated that
accounted for the change in TP concentration and
WBC counts as a function of contamination. The
models were then tested in five additional
horses. Samples were contaminated by one author
and the results presented to another author in
blinded fashion. When noncontaminated TP and
WBC counts were calculated and compared statistically with actual measured values, there was no
significant difference (P ⫽ 0.3 and P ⫽ 0.53, respectively), despite calculated TP concentrations being
0.14 g/L higher and the calculated WBC count being
0.07*109/L lower than measured.
In the original research, several interesting observations were made. First, it took almost 50% blood
contamination (of normal synovial fluid) to raise the
synovial fluid TP concentration toward the reported
septic “cut-off value” of 2.5 g/dL. Adding blood also
raised the neutrophil percentage; however, after the
initial addition, further contamination did not
change the percentage significantly and thus a “corrected” neutrophil percentage could not be calculated.
Second, even in the most heavily contaminated samples, maximum values for synovial TP and WBC
were 3.23 g/dL and 5.03 * 109/L, respectively. This
would indicate that even in blood-contaminated
samples, a total WBC and TP above 20 ⫻ 109/L and
4 g/dL, respectively, would be highly suggestive of a
septic process. Finally, our opinion that it is unlikely that SF harvested in a clinical situation would
be submitted for subsequent laboratory analysis if
⬎10% contaminated because at this point the sample is visually indistinguishable from blood.
The methods presented above are used clinically
to calculate noncontaminated SF TP concentration
and WBC counts from blood-contaminated samples.
However, diagnosing a potential septic arthritis is
Table 2. Formula for Calculation of Uncontaminated White Blood Cell
Count From a Blood-Contaminated Synovial Fluid Sample
Laboratory Values
9
Synovial WBC, *10 /L
Synovial TP, g/L
Blood WBC, *109/L
Blood TP, g/L
Synovial hematocrit, %
Blood hematocrit, %
Calculated values
Uncontaminated TP, g/L
Uncontaminated WBC,
*109/L
Horse Name/Number
3.49
2.90
9.92
7.00
11.10
30.50
1.25
0.78
AAEP PROCEEDINGS Ⲑ Vol. 59 Ⲑ 2013
63
AMBULATORY PRACTICE
still a challenge when results are above normal but
are not above the published cut-off values for septic
arthritis. We believe that in these circumstances,
use of other diagnostic tools, including cytology as
well as the clinical picture and clinician experience,
cannot be overrated.
References and Footnote
1. Bertone AL. Infectious arthritis. In: McIlwraith CW, Trotter GW, editors. Joint Disease in the Horse. Philadelphia: WB Saunders; 1996:397– 408.
2. Bertone AL, McIlwraith CW, Jones RL, et al. Comparison of
various treatments for experimentally induced equine infectious arthritis. Am J Vet Res 1987;48:519 –529.
3. Steel CM, Equine Synovial Fluid Analysis. Vet Clin North Am
Equine Pract 2008;24:437– 454.
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4. Frisbie DD. Synovial joint biology and pathobiology. In:
Auer JA, Stick JA, editors. Equine Surgery. 3rd edition. Philadelphia: Saunders Elsevier; 2006:1036 –1055.
5. Schneider RK. Synovial and osseous infections. In: Auer JA,
Stick JA, editors. Equine Surgery. 3rd edition. Philadelphia:
Saunders Elsevier; 2006:1121–1130.
6. Trotter GW, Mcllwraith CW. Clinical features and diagnosis
of equine joint disease. In: McIlwraith CW, Trotter GW,
editors. Joint Disease in the Horse. Philadelphia: WB Saunders; 1996:120 –144.
7. Schneider RK, Bramlage LR, Moor RM, et al. A retrospective study of 192 horses affected with septic arthritis/tenosynovitis. Equine Vet J 1992;24:436 – 442.
8. Roquet I, Hendrick S, Carmalt JL. The effect of blood contamination on equine synovial fluid analysis. Vet Comp Orthop Traumatol 2012;25:460 – 465.
a
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Redmond, WA 98052– 6399.