Richard A. Savage, MD
product can be used to perform calibration verification for methemoglobin?
Methemoglobin is usually measured with carboxyhemoglobin and oxyhemoglobin
using a multiwavelength oximeter. Absorbance readings at several
wavelengths are combined with preset absorptivities of each hemoglobin
species at each wavelength to calculate the concentration of each
is reported as the sum of oxy-, deoxy-, and carboxyhemoglobin, and
methemoglobin, and the individual species are reported as a percentage
of total hemoglobin. Calibration is possible only for the total
hemoglobin reading because the factors used to calculate concentrations
of individual hemoglobin species from absorbance readings are set
by the manufacturer and cannot be adjusted.
verification needs to be done if there is a calibration for the
analyte in the first place. Since oximeters can be calibrated only
for total hemoglobin, this is the only analyte that requires calibration
A manual method for methemoglobin uses absorbance differences at 630 nm before
and after conversion to cyanmethemoglobin. Again, no calibration is involved,
and calibration verification is not necessary because results are expressed
as a percentage of total hemoglobin without determining the hemoglobin concentration.
Accurate results can be obtained as long as the spectrophotometer exhibits linear
absorbance readings over the range used.
Robert W. Burnett, PhD
Department of Pathology and Laboratory Medicine
Hartford (Conn.) Hospital
Consultant, CAP Chemistry Resource Committee
there established criteria to help laboratories decide which complete
blood cell count results require review? How do laboratories decide
when a manual smear review is necessary? Who should perform manual
The issue of how best to handle the review of CBC results can be
problematic. It’s not easy to use technology and personnel
cost effectively while achieving a reasonable turnaround time and
simultaneously maximizing clinical utility.
cell analyzers can generate results with quantitative or qualitative
abnormalities, or both. They can also generate numerically normal
results, which can reflect a false-positive or false-negative result.
Laboratories have written procedures for such situations that delineate
which abnormalities warrant microscopic review and by whom. Ideally,
these procedures reflect the level of training and experience of
the laboratory personnel, sophistication of the automated blood
cell analyzer, and incidence of disorders in the patient population.
This means that no guideline can be universal or economically feasible.
Identifying the criteria that should be adopted in a given setting
requires the expert judgment of an experienced laboratory director
using input from the clinical physicians and other qualified laboratory
So when should
a laboratory perform a manual peripheral smear review?1,2 PSR might
be warranted for assessing the accuracy of a platelet count, enumerating
or confirming leukocyte populations if automated blood cell results
are unavailable or invalid, and verifying the accuracy of these
results if spurious results are suspected. PSR might also be warranted
for diagnosing specific qualitative abnormalities, including such
hematological malignancies as leukemias, hematological stem-cell
disorders, such as myelodysplasias, and hereditary leukocyte disorders.
And PSR might be warranted for evaluating specific quantitative
and qualitative abnormalities, including cytopenias, hereditary
hemolytic disorders, and for the presence of infectious agents,
as well as for classifying lymphoproliferative disorders.
A related but
secondary issue is who should perform the review.3 In some instances
a bench technologist may suffice, such as when verifying the platelet
count. In other situations, a technologist specialist or other experienced
technologist may be acceptable. Still other cases may require the
laboratory physician’s opinion. This physician should correlate
the numerical and morphological findings with other relevant information
in a particular clinical context, such as when classifying or distinguishing
between infectious agents, such as malaria and babesiosis, or when
initially classifying a newly identified acute leukemia.
Each laboratory must decide who should perform PSR based on its resources and
patient population. In some cases, outside consultations may be required.
- Peterson P, Blomberg DJ, Rabinovitch A, et al. Physician review of the peripheral
blood smear: when and why. An opinion. Lab Hematol. 2001;7:175–179.
- Peterson P. Standard criteria for smear review [letter]. Lab Med. 2002;33:671.
- Ault K. Peripheral smear review: now by whom? Lab Hematol. 2001;7:173–174.
Powers Peterson, MD Department of Pathology
Sentara Virginia Beach (Va.) General Hospital
Member, CAP Hematology/Clinical Microscopy Resource Committee
merged hospitals in our group do automated and manual reticulocyte
counting while others perform only manual counting. Since the results
of manual and automated methods often yield a somewhat different
result, do we need to distinguish between these methods with separate
test identification and reference ranges?
The advent of automated reticulocyte counting using various fluorescent
dye/laser excitation combinations has created a group of tests that
have criteria for identifying a red cell as a reticulocyte, which
clearly differs from the criteria applied when identifying reticulocytes
visually in new methylene blood-stained smears. Therefore, it is
not surprising that manual and automated methods do not agree entirely.
literature and CAP reticulocyte counting Surveys have demonstrated
that automated methods using fluorescent dye/ laser excitation yield
higher values than manual methods. Consequently, manual reticulocyte
counts and automated reticulocyte counts should be treated as different
tests and reported with a reference range derived specifically for
In reviewing this issue with members of the CAP Hematology/Clinical Microscopy
Resource Committee, I learned that all of the members who perform automated
and manual testing maintain separate tests and reference intervals.
MD Department of Pathology
Children's Hospital Medical Center of Akron (Ohio) Chair,
CAP Hematology/Clinical Microscopy Resource Committee