CBC, serum chemistry profile, and urinalysis
CBC findings in patients with IMPA include a neutrophilic leukocytosis (24% to 69% of patients), leukopenia (8% to 24% of
patients), mild nonregenerative anemia (15% to 35% of patients), and mild thrombocytopenia (8% to 27% of patients).5,6,8,9,15 Other findings in patients with IMPA include an elevated alkaline phosphatase activity (10% to 60% of patients), hypoalbuminemia
(7% to 27% of patients), and an elevated urine protein:creatinine ratio (7% to 30% of patients).5,6,8,9,15
Findings from a CBC such as a neutrophilic left shift, severe anemia, severe thrombocytopenia, evidence of hemolysis, or serum
chemistry results consistent with renal or hepatic disease should prompt your suspicion of other infectious or multiorgan
Perform an ANA test in dogs with multiorgan disturbances. Most dogs with IMPA have negative ANA results,16 but a positive result should increase your suspicion of SLE.33 To help confirm a diagnosis of SLE, a lupus erythematosus cell preparation can be performed, but an ANA test is preferred
because of increased sensitivity and availability through reference labarotories.13
5 & 6. Dorsopalmar and lateral carpal radiographs demonstrating lesions typical of an advanced case of erosive IMPA in a dog.
Note the intracapsular swelling, collapsed joint spaces, subchondral bone erosions, and periarticular osteophytosis.
Obtain joint radiographs to rule out other causes of lameness and joint pathology and to characterize IMPA as erosive or nonerosive
(Figures 5-8). Radiographs typically demonstrate joint effusion and periarticular soft tissue swelling with IMPA.1,2,6,15 On occasion, joint effusion is not detected.5
In advanced cases of erosive IMPA, subchondral bone erosions, subluxations, luxations, or ankylosis is present.29,30 If you suspect erosive disease, obtain stressed medial, lateral, flexed, and extended radiographic views of the carpus and
tarsus as they are useful in detecting joint instability. Early in the disease process, radiographs of both erosive and nonerosive
IMPA may appear similar with no obvious erosion.30 Thus, later in the disease process, you may need to repeat radiographs to determine the true form of disease.
7 & 8. Lateral and dorsopalmar carpal radiographs in a dog with intracapsular swelling (effusion) typically seen with nonerosive
Antigen and antibody tests
Vector-borne infectious polyarthritis, particularly in disease-endemic areas, should be ruled out with appropriate antibody
titers (initial and convalescent) and antigen detection methods (PCR, immunohistochemistry). Rickettsial organisms, including
Borrelia burgdorferi, Rickettsia rickettsii, Ehrlichia canis, and Anaplasma phagocytophilum, and other vector-borne agents such as Leishmania species cause a polyarthritis that can be difficult to differentiate from IMPA.1,32 A recent study demonstrated a significantly greater percentage of dogs with IMPA were seropositive for B. burgdorferi than dogs in the general hospital population and suggested that some cases of IMPA were due to Lyme disease.5 Because of difficulty in completely ruling out some vector-borne diseases, assessing response to an antibiotic trial may
be warranted before initiating immunosuppressive therapy.
Muscle and synovial biopsy, CSF analysis, and rheumatoid factor test
Perform a muscle biopsy in cases of suspected polyarthritis-polymyositis syndrome.1,20 With this syndrome, muscle biopsy samples demonstrate inflammatory infiltrates, often focal, with varying degrees of atrophied
muscle fibers and areas of necrosis.20 A synovial biopsy, while usually not indicated in patients with acute disease, can be useful in diagnosing erosive IMPA
before radiographic changes are evident.29,30 With erosive IMPA, synovial biopsy samples demonstrate villous hypertrophy of the synovial membrane and extensive infiltration
of mononuclear cells into the synovium.28,30,34 CSF analysis should be performed in cases of suspected polyarthritis-meningitis syndrome.10,19 In these cases, typical CSF findings are a normal to moderately elevated protein level and a neutrophilic or mixed leukocyte
pleocytosis.10 A rheumatoid factor test is not a specific or sensitive test for diagnosing idiopathic erosive IMPA in dogs.6