Diagnosis of leishmaniasis in dogs should be based on an integrated approach considering signalment, history, clinical findings and results of basic laboratory analyses that detect the parasite or evaluate the immune response of the host. Any canine breed can be affected by leishmaniasis, although some breeds (e.g., German Shepherd Dog and Boxer) seem to be more predisposed to overt disease than others. Dogs can be infected at any age, but the prevalence of infection within certain age groups has a bimodal distribution, with a first peak in dogs < 3 years of age and a second in dogs from 8–10 years of age. Because disease distribution varies widely throughout the world, it is essential to find out whether a dog lives in or has travelled to known endemic areas and hence has been potentially exposed to sandfly vectors. It is also important to find out whether the dog received preventive treatments that are potentially effective against sandflies, or whether treatments were administered that may interfere with immune system efficiency.

The most common clinical signs of leishmaniasis in dogs are lymph node enlargement and skin lesions. However, a broad and heterogeneous spectrum of clinical signs and lesions can be detected during physical examination. Basic tests include a complete blood count (CBC), serum biochemical analysis, serum protein electrophoresis and urinalysis. In leishmaniasis, these tests allow the detection of one or more of changes associated with the disease. However, in clinics, specific diagnostic methods are grouped into two main categories: direct (cytological evaluation, histological evalution and polymerase chain reaction (PCR) assay) and indirect (serology).

As a general rule, leishmaniasis in dogs can be quickly and efficiently confirmed by cytological, serological or PCR analyses in dogs with overt clinical signs or severe alterations of relevant clinicopathological variables. However, this is not the situation for most dogs living in endemic areas that undergo periodic medical visits and often have early vague and non-specific signs. The main problem is the demonstration of a cause-effect relationship between the direct or indirect test results and the observed alterations. Without such confirmation, there is a risk of concluding that the disease is present when actually it may not be. In dogs with a history and clinical signs suggestive of leishmaniasis, the first-line diagnostic approaches should include cytological analysis of affected tissues and specific serology assays.

Cytological examinations should involve fine-needle aspiration of the following tissues or lesions: papular, nodular, and ulcerative skin lesions (ulcerative lesions can also be evaluated via impression smears); bone marrow and lymph nodes when clinical signs (e.g., anaemia and lymphadenopathy, respectively) suggest their involvement; any biological fluids that can be obtained from affected sites (e.g., synovial fluid when arthritis or polyarthritis is present or cerebrospinal fluid (CSF) when neurological signs are present). But, in the absence of clinical signs that can be attributed to, or that involve, a particular organ or tissue, samples should be obtained from tissues in which parasites are more likely to be detected such as bone marrow, lymph node, spleen and buffy coat from peripherally obtained blood, in descending order of diagnostic sensitivity.

When tissues do not have cytological evidence of Leishmania infection, serological testing becomes crucial to decide whether a dog is sick because of leishmaniasis. A high antibody titre will confirm the disease is present. On the contrary, a low antibody titre generally suggests that the dog is not diseased due to leishmaniasis; hence, the dog could be affected by a different disorder that shares similar signs. In dogs with a low antibody titre, other diagnostic procedures, chosen on the basis of the clinical signs, should be followed.

In some situations when non-cutaneous lesions (e.g., systemic signs) exist that are highly suggestive of leishmaniasis, a PCR assay must be performed on tissues such as bone marrow or lymph nodes or whole blood because the PCR test provides a good chance of detecting the DNA of the invading parasite.

Leishmania infantum infection may develop over a period of a few weeks to several months toward clinical pictures that can be highly variable, and therefore it is not always easy to classify dogs as infected or perhaps as affected. Based in the results of the previous diagnostic test described, one of the classification systems published proposes classifying dogs with positive results of serological tests or in which the parasite has been identified via direct diagnostic methods into four clinical stages:

 A - exposed dogs

 B - infected dogs

 C - sick dogs (dogs with clinically evident leishmaniasis)

 D - severely sick dogs

The proposed classification system should not be considered as a rigid and schematic outline for such a complex disease but as a useful tool for management and treatment of affected dogs.