Explanation of how the ANA titer is obtained
I don't understand how the titer could be positive but below detectable
limites... doesn't make sense to me.  The sample needs to be diluted or it
doesn't... dilution determines the titer.

Anti-Nuclear Antibodies (ANA) are antibodies which react with nuclear
components of cells. Among the nuclear antigens to which antibodies react
are DNA, RNA, nucleoli, histones and non-histone proteins.
Found in patients with a variety of diseases, ANAs also occur in low titers
in about 5% of the normal population, with higher prevalence in the elderly.
Most patients with active systemic lupus erythematosus (SLE) have positive
ANAs, usually of high titer (1:160 or greater). ANAs often appear also in
other rheumatic diseases, and occasionally in a variety of other conditions.
If a positive ANA is present, further testing may be indicated to identify
the antibody present. Since specific anti-nuclear antibodies occur in
various diseases, follow-up testing may be indicated, depending upon the ANA
titer, the clinical suspicion and the pattern of fluorescence in the ANA
test. For example, a high titer ANA speckled pattern may be associated with
antibodies to extractable nuclear antigens (ENA). The ENA antibody test may
be helpful if connective tissue diseases are suspected, and aid in
differentiation between various diseases in this category.
By ordering the ANA Reflexive Panel, follow-up tests (DNA-Binding and ENA
Screen/Identification) are performed on positive samples, and the test for
anti-SSA antibodies is also done. For a listing of antibodies with ANA
patterns, disease associations, and suggested tests to order, see the above
chart.

METHOD DESCRIPTION:
Indirect immunofluorescence is the method used to screen for ANA. All
samples are tested on two substrates: HEp-2 tissue culture cells (with
enlarged nuclei and numerous dividing cells), and rat liver tissue, which
has been the substrate classically used for ANA testing. The patterns of
fluorescence (homogeneous, speckled, nucleolar, rim or centromere) are
reported from both cell types. The titer is reported on rat liver tissue.
The screen on rat liver is performed at a dilution of 1:40, and positives
titered to 1:640. Further titering on rat liver or HEp-2 cells will be done
at the physician's request.

REFERENCE RANGE: Age 40 yrs=1:40 or Neg. Age >40 yrs=1:80 or Neg.
About 5% of a normal reference population has a positive ANA test. Most of
these have only low ANA titers, i.e. 1:40. A higher prevalence of positive
ANA tests is seen in older, apparently normal populations, especially in
females. About 15% of women over age 50 have positive ANA tests. Most of
these are positive at low titers (1:40), but some older women have ANA
titers as high as 1:160 .