Date: Mon, 17 Apr 1995 14:17:27 +0500
From: ghmcleaf{CONTRACTOR/ASPEN/ghmcleaf}%NAC-GATEWAY.ASPEN@ace.aspensys.com
Subject: CDC Nat. AIDS Hotline Training Bul. #135


           CENTERS FOR DISEASE CONTROL AND PREVENTION
                       HIV/AIDS PREVENTION
           CDC NATIONAL AIDS HOTLINE TRAINING BULLETIN
.................................................................

April 4, 1995                                               #135

These are answers from the Centers for Disease Control and
Prevention (CDC) to a question submitted by the CDC National AIDS
Hotline concerning CD4+ counts.

The 1993 revision of the AIDS case definition includes
HIV-infected persons with a CD4+ percent less than 14.  What does
a CD4+ percent less than 14 mean?  CD4+ as a percent of what?
What is the range of normal CD4+ percents?

The most commonly used measure of immune function in HIV-infected
persons is the enumeration of CD4+ cells.  Currently, the
enumeration of these cells is the best assessment that can be
made easily and economically.  Active research continues in the
area of development of better measures of immune function.

CD4 is the name given to a specific molecule found only on the
surface of T-helper lymphocytes and a very few other human cells.
White blood cells are routinely enumerated on the blood test
commonly called "CBC" (complete blood count).  The various types
of white blood cells are routinely reported as well (neutrophils,
lymphocytes, mononuclear cells, eosinophils, and basophils).
CD4+ cells are T-helper lymphocytes.  CD8+ cells are T-suppressor
lymphocytes.  Enumeration of subsets of lymphocytes requires a
higher degree of laboratory sophistication and expense than that
needed for a CBC (one machine used for such enumeration is the
fluorescent activated cell sorter or FACS).

CD4+ T lymphocytes are the cells primarily responsible for
initiating the body's immune response to various microbes.  This
cell is also the primary cell in the body that HIV attaches to
and infects.  Decreases in percentages and absolute numbers of
CD4+ T cells are an indication that the immune system has been
compromised and can no longer protect the body from bacteria,
viruses, and other microbes that can cause infections.  The
percentage of CD4+ cells is the proportion of lymphocytes that
are CD4+ cells versus CD8, CD56, etc.  The absolute number of
CD4+ cells is derived by multiplying the percentage of CD4+ cells
by the number of circulating lymphocytes in the blood.  The
expected normal range for the percentage of CD4+ cells in whole
blood ranges from 33% to 60%, and the normal range for the
absolute number of CD4+ cells is usually between 500 and 1600
cells per microliter of blood.  These ranges may differ from
laboratory to laboratory performing the tests.



Compared with the absolute CD4+ cell count, CD4+ percentages are
less subject to variation on repeated measurements.  However,
data correlating the natural history of HIV infection with CD4+
percentages have not been as consistently available as data on
absolute cell counts.  Therefore, CDC emphasizes the use of
absolute numbers but allows for the use of CD4 percentages.

The percentage of CD4+ T lymphocytes in a whole blood sample is
measured by using a method called flow cytometric
immunophenotyping.  Guidelines for laboratories conducting this
test were published in the Morbidity and Mortality Weekly Report
(March 4, 1994, Vol. 43, RR-3).