9.3 Parameter Limitations (continued)
The red blood cell dilution contains all the cellular elements of the blood: RBC, WBC, and PLT. Platelets are
not counted since the size falls below the discriminator threshold. Leukocytes are included in the RBC
count, but since the ratio of RBCs to WBCs is approximately 1000:1, the introduced WBC count is almost
negligible. Exceptions are noted below.
Measurement of high RBC levels is influenced by coincidence factors (e.g. counting of two cells as one)
which may produce falsely low results. The instrument is compensated for this effect by an algorithm to
produce a linearity range according to the specifications
Leukocytosis with
concurrent anemia
In samples where the WBC is very high and at the same time the RBC is low, the
WBC may cause a false increase in the RBC count. The WBC is always included
in the RBC count, but the contribution is not significant under normal
circumstances. The RBC count may be corrected by simply subtracting the WBC
from RBC.
Agglutinated Red Blood
Cells
This might cause a falsely decreased RBC count. Blood samples containing the
agglutinated red blood cells may be identified by observing abnormal MCH and
MCHC values, as well as by examination of the stained blood film.
IgM immunoglobulins which are elevated in cold agglutinin disease may lower
RBC and PLT counts and increase the MCV.
The red cell distribution width is a function of the RBC count and derived from the RBC histogram. In most
cases, any error introduced in the MCV may also cause the RDW to be erroneous.
Blood transfusions may raise the RDW significantly due to the presence of bi-
modal populations.
Measurement of high WBC levels is influenced by coincidence factors (e.g. counting of two cells as one)
which may produce falsely low results. The instrument is compensated for this effect by an algorithm to
produce a linearity range according to the specifications.
WBC in concentrations that exceeds the linearity limits of the system will require
dilution of the blood sample. Re-assaying the diluted sample will help to obtain
the correct assay value.
Nucleated Red Blood
Cells, NRBC
Immature, nucleated red blood cells are large and not lysed like mature RBCs,
thus they will be classified as a WBC and may cause falsely elevated WBC and
lymphocyte results. If the number of the NRBC is sufficient to activate the DE
alarm, such interference will be detected. An overview of a stained blood film
can reveal the presence of NRBCs.
In particularly rare instances, the RBC in the blood sample may not completely
lyse like expected. These non-lysed cells may be detected on the WBC histogram
with a DE alarm, or as an elevated baseline on the side of the lymphocyte
population. Non-lysed RBCs will cause a falsely elevated WBC and lymphocyte
count. (See also NRBC above)
Hemolyzed specimen contains red cell debris, which may falsely elevate the
WBC and/or PLT count. Hemolysis can be detected by looking at the color of the
plasma in an EDTA-sample that has been allowed to sediment.
This disease state may result in a spurious low WBC count, if the leukocytes are
more fragile than normal and becomes destroyed in the sample. The cell
fragments will also interfere with the WBC differential parameters (LYM, GRAN
and MID). A falsely low WBC count may also be seen in patients with
lymphocytic leukemias due to the presence of abnormally small lymphocytes,
which may not be counted by the instrument.
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