Using titers to diagnose disease: when is a positive a positive? (Proceedings)
For most diagnostic tests, the 'titer' is the minimum dilution of a substance that is required to yield a positive result. For example, for detection of anti-Leptospira sp. Antibodies, the titer is the dilution of serum which still causes microbes to crosslink. Unfortunately, the sensitivity and specificity of the available tests varies for each organism, for each laboratory, and even from day-to-day due to observer error. Regardless of the test being performed it is rare that a 'positive' result in and of itself is a guarantee of disease. This presentation will review the more common 'titer' tests available to us as clinicians, and discuss common methods of interpretation (and misinterpretation).
Antibodies are produced by B-cells and plasma cells. Initially IgM is rapidly produced to bind and neutralize antigens, fix complement, and opsonize microbes for phagocytosis. Once T-cells are recruited and recognize the activated B-cell, isotype switching to IgG or IgA occurs. Antibodies are then produced in large amounts and enter the systemic circulation. For unknown reasons some organisms are able to produce fulminant infections without induction of a significant antibody response; this likely occurs because they induce alternative immune responses that rely less on antibodies as effector molecules. After resolution of infection antibody concentrations may or may not decline over time, which may depend on whether the organism persists in the host.Leptospirosis
The gold standard for diagnosis of leptospirosis is identification of organisms in urine or in tissues. Unfortunately these methods are very insensitive (organisms are rarely visible despite the presence of infection), invasive (renal biopsy), and not readily available (dark field microscopy). Therefore antibody agglutination titers are routinely used in conjunction with consistent clinical signs to diagnose cases. Agglutination tests detect antibodies by testing the ability of the patient's serum to cause intact Leptospira organisms to be crosslinked by circulating antibodies: when viewed with a microscope, crosslinking results in visible agglutination. Serum is tested against laboratory strains of multiple serovars in order to provide titers for each one. Unfortunately, the Leptospira spp. vaccines uses killed whole organisms to induce immunity, and as a result the antibodies that are produced are against multiple proteins, and cannot be distinguished from infection-induced titers by the agglutination test. Additionally infection results in cross-reacting antibodies against multiple serovars. Therefore, interpretation of Leptospira titers must be done with caution. In general:
1. The highest serovar titer is considered to be the infecting serovar. This rarely is clinically important, with the exception of those cases where hepatic failure is also present. Serovars icterohemmorhagica and canicola are most commonly associated with acute liver failure, whereas grippotyphosa has been associated with chronic hepatopathy.
2. The Leptospira spp. vaccines usually cause relatively low antibody titers against all serovars; the highest serovars may be the vaccinal ones, although this is not always true. Regardless, although titers may be high soon after vaccination (i.e. greater than or equal to 1:800), they frequently decrease to much lower concentrations.
3. A titer only indicates exposure or vaccination, and is not synonymous with disease. In the absence of renal dysfunction there is no indication that treatment is needed.
4. Infected dogs usually have high titers at the time of diagnosis. However in rare cases astute clinicians may test dogs early in their disease process, at which time the antibody titer may be low or negative. A convalescent titer may be required after a 10-14 day period to confirm a 4-fold increase in titer.
Feline enteric coronavirus is a common cause of transient diarrhea in kittens, although asymptomatic infection is likely just as common. In rare cases the coronavirus mutates and disseminates via the host's macrophage-phagocytic system, and infection results in the wet or dry forms of feline infectious peritonitis. At this time published data does not support the claim that any antibody test is capable of differentiating between the mutated and non-mutated viruses. As a result there is no such thing as a positive 'FIP' titer, only a positive coronavirus titer. There is no single mutation that causes FIP. Research has implicated mutations in two different viral proteins (3C and 7B) as being associated with the FIP viral phenotype. However mutations can occur in any site within these proteins, and thus attempting to detect antibodies against a single mutated form of these proteins is at this time not realistic. The only practical use to the coronavirus titer is as a screening test—negative patients are very unlikely to have FIP (although even this is not always true), whereas positive patients are of unknown status.
Histoplasmosis, blastomycosis, aspergillosis
Histoplasma capsulatum, Blastomyces dermatiditis, and Aspergillus sp. are widespread in the environment. Therefore it is reasonable to assume that many animals are exposed and develop antibody titers without developing disease; the specificity of these tests is thus relatively low. Conversely, many animals infected with these fungal agents never develop antibody titers that are measurable using existing assays, therefore the sensitivity is poor as well. Treatment for any of these diseases should never be instituted solely on the basis of titer results; in fact most internists no longer request H. capsulatum, B. dermatiditis antibody titers at all. Studies as to the usefulness of Aspergillus sp. titers are conflicting; a recent retrospective reported that this test may be more sensitive and specific than previously thought, although again internists will not treat solely based on the results of this test.