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466 BASIC SKILLS IN INTERPRETING LABORATORY DATA
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a
METHODS COMMONLY USED FOR DETECTION
OF VIRUS
EXAMPLES OF SPECIES
INFECTING HUMANS
NUCLEOCAPSID,
SYMMETRY
Hemorrhagic fever virus
Hantavirus
Helical La Crosse virus
Yes Helical Lassa mammarenavirus
+/−
circular
FAMILY NATURE ENVELOPE SHAPE
TABLE 19-7.
Arenaviridae
Human astrovirus
+
linear
Astroviridae
Yes
linear
Bunyaviridae
+
linear
Caliciviridae
CHAPTER 19 • InfECTIous DIsEAsEs: fungI, VIRusEs, AnD MyCobACTERIA 467
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continued)
Helical
Yes
+
linear
syndrome- related coronavirus
related coronavirus
syndrome-
related coronavirus
syndrome-
δ virus
Yes Helical
circular
Helical
Yes
linear
Yes
+
linear
Dengue virus
Coronaviridae
b
Deltavirus
Filoviridae
Flaviviridae
468 BASIC SKILLS IN INTERPRETING LABORATORY DATA
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a
METHODS COMMONLY USED FOR DETECTION
OF VIRUS
diagnosis)
EXAMPLES OF SPECIES
INFECTING HUMANS
NUCLEOCAPSID,
SYMMETRY
+
linear
Yes Helical
linear
TABLE 19-7.
FAMILY NATURE ENVELOPE SHAPE
Herpesviridae
Orthomyxoviridae
CHAPTER 19 • InfECTIous DIsEAsEs: fungI, VIRusEs, AnD MyCobACTERIA 469
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continued)
immunity)
enteroviruses)
Yes Helical
linear
linear
+
linear
Yes Helical
linear
linear
Paramyxoviridae
Picobirnaviridae
Picornaviridae
Pneumoviridae
Reoviridae
470 BASIC SKILLS IN INTERPRETING LABORATORY DATA
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a
METHODS COMMONLY USED FOR DETECTION
OF VIRUS
Antigen- antibody combination tests reduce seronegative
EXAMPLES OF SPECIES
INFECTING HUMANS
NUCLEOCAPSID,
SYMMETRY
Yes
reverse
transcribing
dimer
Helical Rabies lyssavirus
Yes Bullet
linear
Alphavirus
∼
Yes Arboviruses including
+
linear
Rubella virus
FAMILY NATURE ENVELOPE SHAPE
TABLE 19-7.
Retroviridae
Rhabdoviridae
Togaviridae
+
a
b
Source
CHAPTER 19 • InfECTIous DIsEAsEs: fungI, VIRusEs, AnD MyCobACTERIA 471
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Once the sample is collected, it should be promptly transported to the laboratory in a sterile, leak- proof container using
the appropriate viral transport media to maximize viral recovery. Every eort should be made to prevent delay between the
time of specimen collection and its arrival to the laboratory.
When delays are expected, viral samples should be refrigerated
at 4°C or frozen at –70°C. Subsequently, the laboratory will need
to follow specic processing procedures for each specimen and
the dierent diagnostic viral test methodologies.
e laboratory techniques used in the diagnosis of viral
infections include cell culture, cytology and histology, electron
microscopy (EM), antigen detection, NAATs, and serologic
1,13,32-38
testing.
e choice of test(s) varies depending on the
clinical syndrome or disease, virus(es) involved, patient characteristics, collection site, purposes of the test (eg, screening,
diagnosis, conrmation or monitoring), time to result, laboratory capabilities/sta expertise, and cost. e following section,
as well as Tables19-6 and 19-7, provides a brief summary of the
common methods currently used in diagnostic testing of common viruses.
1,13,31,32
For more detailed information, the reader
is referred to current published literature, standard reference
books, and the latest edition of reference manuals (eg, Manual
of Clinical Microbiology). A list of virology services oered by
the CDC can be found on the CDC website (https://www.cdc
.gov/laboratory/specimen- submission/list.html).
Cell Culture
e use of cell culture rapidly expanded the knowledge about the
epidemiology, clinical characteristics, and diagnosis of common
viral infections in the 1950s and 1960s. Subsequently, the use of
cell cultures to isolate a virus became the gold standard method
for the diagnosis of viral infections in most clinical virology
laboratories for the next 50 years.
technologies for viral detection were slowly introduced in the
clinical laboratories, including enzyme immunoassays, IgM class
capture assays, rapid centrifugation cultures, direct viral antigen
detection from clinical specimens by immunouorescence, and
monoclonal antibodies for identication. However, rapid and
accurate serology and molecular methodologies have become
cornerstones for virus detection and identication in clinical
laboratories during the past decade, resulting in a decline in the
use and prominence of cell cultures in larger academic medical centers and tertiary- care facilities. Despite these changes in
routine diagnostic virology, viral cultures play an important role
in the discovery of new or unknown viruses, identication of
variants of known viruses, detection of drug- resistant viruses,
typing of serologic strains, detection of viruses in special patient
populations (eg, immunocompromised patients), research and
development of antiviral drugs and vaccines, and performance
of viral susceptibilities.
34-36
e advantages of cell culture include good specicity and
sensitivity, the capability of detecting multiple viruses if present,
and the cultivation of the virus for further laboratory testing (eg,
susceptibility testing, serologic strain typing), if needed.
Cell cultures can be useful when combined with highly specic
monoclonal antibodies or engineered cell lines (eg, to produce
virus- induced enzymes), especially if the cost of other testing
13,32-36
During that time, other
32,34-36
methods is greater than cell cultures or when the clinical laboratory does not have the ability and equipment to perform molecular detection methods. e disadvantages of cell culture include
the long time needed for the detection of viruses using conventional cell culture (eg, days to weeks), the need for cell culture
facilities, the expense of performing cell culture, and the fact
that the methodology is not applicable to all viruses (eg, viruses
that have not grown in conventional cell cultures [ie, Group C
rhinovirus]). is greater demand for technical laboratory experience with cell cultures, need for comprehensive quality control program, and strict procedures for handling biohazardous
materials in the clinical virology laboratory are being replaced
by rapid and sensitive antigen screening assays and NAATs.
Several dierent types of cell culture are available to grow
clinically important viruses.
32,34-36
Each virus requires a predened cell line, which is established once a cell culture has
been subcultured invitro (the reader is referred to a comprehensive list of available cell lines and virus susceptibility proles34). e dierent types of cell lines can be divided into three
categories: primary, diploid (also called low passage cell lines),
and heteroploid. Primary cell lines (eg, rhesus monkey kidney
[RhMK] cells or human amnion cells) are prepared from animal
or human tissues and can withstand only one or two passages
until the cells die. Diploid cell lines are usually derived from
fetal or newborn cells (eg, human embryonic lung broblast
lines such as WI-38 or MRC-5) and can undergo 20 to 50 passages before cells are unable to survive. Continuous cell lines
can undergo an indenite number of passages without reducing the sensitivity to virus infection. Heteroploid cell lines are
characteristically derived from human or animal cancers (eg,
human epidermoid lung carcinoma [HEp-2, HeLa]) or are cells
transformed invitro (eg, LLC-MK2). Heteroploid cell lines can
also include genetically engineered cells (eg, ELVIS cell mixture
for the detection of herpes simplex virus [HSV] types 1 and 2).
Most specimens are inoculated onto two or more cell lines (eg,
RhMK, MRC-5, HEp-2) based on the most likely viruses associated with the type of clinical specimen that was submitted.
e growth of a virus from a clinical specimen provides direct
evidence that the patient was infected with a virus. e main
method for detecting growth from the cell culture method is by
microscopic examination of the unstained cell cultured monolayers for morphologic changes or cytopathic eect (CPE).
e characteristics of the CPE (eg, which cell culture types were
aected; what is the resultant shape of the cells; whether the
eect is focal or diuse; the time of its appearance and progression) can be used for primary and denitive identication of
the virus. Subsequently, direct and indirect uorescent antibody
(DFA and IFA, respectively) staining of cells with virus- specic
monoclonal antibodies harvested from the culture is oen used
to conrm the identication of the virus (the reader is referred
to a comprehensive list of available DFA and IFA reagents and
target virus to detect34). Molecular or ancillary traditional testing can alternatively be used for viral identication.
Some viruses, such as inuenza, parainuenza, and mumps
virus, grow in cell cultures without producing CPE so that other
methods are used to identify and detect these viruses, including hemadsorption and interference.
32,34-36
Hemadsorption
34
32,34-36
472 BASIC SKILLS IN INTERPRETING LABORATORY DATA
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involves the removal of the culture medium from the inoculated cell culture, adding a suspension of erythrocytes, and
examining for hemadsorption with a low- power microscope
as manifested by adherence of the red cells to the cell culture monolayer due to the presence of a hemadsorbing virus.
Hemadsorption is used to detect these viruses, which can grow
rapidly and reach high titers in cell cultures without producing CPE. Used to detect viruses such as rubella, interference
involves growing a virus that yields a cell culture resistant to
other viruses (to which it is normally susceptible). e viruses
that produce hemadsorption or interference subsequently can
be identied by staining with virus- specic monoclonal antibodies or antiserum.
Shell vial cultures with centrifugation and pre-CPE detection
are used to decrease the amount of time required to grow a virus
by conventional cell cultures.
32,34-36
is technique makes use of
cells grown on microscope coverslips that are placed within shell
vials and covered with culture media. Aer cultures are incubated for 1 to 3 days, FA staining is performed on the cells on
the coverslips to recognize an antigen in the nucleus of infected
cells. Shell vial cultures have been commonly applied for the
detection of cytomegalovirus (CMV), HSV, varicella- zoster
virus (VZV), enteroviruses, and the human respiratory viruses.
Centrifugation- enhanced rapid cell cultures can also be used
with cocultivated cells (eg, mixture of two cell lines together) or
genetically engineered cells (eg, ELVIS [enzyme- linked virusinducible system], BGMK- hDAF [bualo green monkey kidney cell line]) for the rapid identication (eg, 16 to 72hours)
and blind staining of multiple viruses from a single shell vial or
tray well.
32,34-36
Molecular Diagnosis
e detection of specic viral NAs by molecular diagnostic techniques is revolutionizing the eld of diagnostic virology.
NAATs have become the “gold standard” in clinical virology
laboratories and are replacing older techniques such as cell cultures for detecting clinically signicant viruses. Many dierent techniques are used in viral NA detection, including direct
hybridization assays, target (template) amplication (eg, PCR,
self- sustained sequence replication method, strand displacement amplication), and signal amplication (eg, branchedchain DNA [bDNA] assay and hybrid capture assay). Among
these, PCR has been the most important technique in diagnostic
virology because of its versatility in detecting DNA or RNA and
being able to provide qualitative and quantitative information
on specic viral NAs.
e use of NA detection has become the standard of care
(eg, hepatitis C virus [HCV] and HIV) or the test of choice for
routine diagnosis of many viral infections (eg, bocaviruses, HSV
central nervous system [CNS] infections, human HVS 6 and 7,
human metapneumovirus, human papillomavirus [HPV]).
e FDA has cleared or approved commercial molecular detection assays; several viruses, including hepatitis B and C viruses
(HBV, HCV); HIV; HSV; CMV; adenovirus; avian u; enteroviruses; inuenza; and HPV. An FDA- approved simple multiplex
PCR test (eg, xTAG Respiratory Viral Panel) is also available for
rapidly screening common respiratory viruses (eg, respiratory
1,13,32,34,38
1,13,32
syncytial virus [RSV], inuenza A and B, adenovirus) or sub-
1,13,38-41
types.
In addition, FDA- approved, high- throughput,
syndromic viral tests are also available for detection of GI and
CSF pathogens (eg, BioFire FilmArray GI and Meningitis/
Encephalitis panels).
1,39-43
An up- to- date listing of cleared or
approved nucleic acid diagnostics tests is available at the FDA
website (https://www.fda.gov/medical- devices/vitro- diagnostics
/nucleic- acid- based- tests).
e advantages of viral NA detection methods include
the rapidity of results (eg, hours for real- time PCR and one
to several days for other methods), maximal sensitivity for
virus- specic detection and identication, adequate to excellent specicity, dramatic increase in availability of commercial
assays, the ability to detect viruses that are dicult to culture,
and the ability to detect NAs without viable virus present in
the clinical specimen. Historically, equipment and reagents
costs, service contracts, and technical expertise have been the
major barriers to implementing molecular testing. However,
NAAT has rapidly evolved and allows viruses such as inuenza virus and RSV to be detected within 20 to 30 minutes
at near POC and with sensitivity similar to other laboratory
testing methodologies.
1,13,39-41,44
Several devices (eg, FilmArray
Respiratory Panel EZ assay [Biolm, Salt Lake City, UT]; ID
NOW RSV [Abbott Diagnostics, Scarborough, ME]; the cobas
Liat Inuenza A/B & RSV [Roche Diagnostics, Indianapolis,
IN]) have been waived by the Clinical Laboratory Improvement Amendments (CLIA).
13,40,41,44
Molecular assays have
become the standard of care for diagnosing viral infections
and monitoring antiviral therapy and patient outcomes.
Cytology and Histology
Cytopathologic eects (CPEs) on cells are produced by many
viruses. Cytologic examination can be performed on smears
prepared from samples that are applied to a microscope slide
or “touch preps” of unxed tissues.
suggestive of a viral infection and provide identication of cell
morphologies (eg, “owl’s eye” nuclear inclusions consistent with
CMV), cell lysis, or other cell changes (eg, vacuolation, syncytia,
inclusion bodies). e specic virus cannot be identied unless
virus- specic immunostaining techniques are used. Applications of cytology to viral diagnosis include the Tzanck smear with
Giemsa reagent for demonstrating the presence of HSV or VZV
infection, Papanicolaou staining of cells obtained from the uterine
cervix (Pap smear) for providing evidence of HPV infection, and
cytologic staining of urinary sediments for screening the presence
of either CMV or polyomaviruses JCV and BKV.
Similar to cytology, histologic examination of tissue provides
evidence to suggest a group of viruses that may be causing infection, but it does not identify a specic virus.
shortcoming, histopathology has been useful in dierentiating
between asymptomatic viral shedding and clinically important
infections of CMV and has been used for the diagnosis of CMV
infections in tissue samples obtained from biopsy or at autopsy.
In addition, detection of specic viral antigens by immunohistochemistry and detection of specic viral NAs by insitu hybridization (ISH) or PCR has allowed specic viruses to be identied
by histopathology.
32,34-36
Cytologic ndings are
32,34
Despite this
1,13,32
CHAPTER 19 • InfECTIous DIsEAsEs: fungI, VIRusEs, AnD MyCobACTERIA 473
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Electron Microscopy
Viruses are the smallest infectious pathogens that range in diameter from 18 to 300 nm.
a light microscope can be performed only on pathogens with a
diameter >200 nm. Electron microscopy (EM) allows visualization of characteristic viral morphology and, unlike direct detection or molecular methodologies, is capable of detecting the
distinctive appearances of multiple viruses, if present.
considered the most useful routine test for poxviruses.66 Diagnostic virology laboratories also commonly use EM for detection
of viruses that are not detected with cell cultures or other methods (eg, gastroenteritis viruses such as noroviruses, coronaviruses, astroviruses, enteric adenovirus, and calicivirus).
Several techniques have been incorporated to allow the
visualization of viruses with EM from various types of clinical
specimens. Negative staining is a technique for identication of
viruses in uid samples, stool samples, and blister uid. in
sectioning can be performed on tissue samples that have been
xed with specic xatives for EM study, and it can be used
to visualize herpesviruses, respiratory viruses, and rabies virus.
More sensitive methods are replacing the routine use of EM
for detecting clinically signicant viruses.
of EM is its economical, quick (eg, same day), adaptable, and
straightforward approach for detecting viruses. e major disadvantages of EM include poor sensitivity, initial equipment
expenses, and need for highly skilled laboratory sta.
32,37
Direct visualization of a virus with
32,37
13,32,34,37,43
13,32,37
An advantage
EM is
Direct Antigen Detection
Antigen detection methods involve the use of virus- specic
antibodies directed toward viral antigens in a clinical speci-
1,13,32,38
men.
antigen detection include RSV, inuenza virus, parainuenza
virus, adenovirus, HSV, VZV, CMV, rotavirus, HBV, and measles
virus.
the rapidity of diagnosis (eg, several hours to 1day), usefulness
for the identication of viruses that are dicult to culture, and
detection of viral specic antigens even if viable virus is not
present in the clinical specimen. e disadvantages include the
potential for false- positive and false- negative results, diculty
of performing batch testing, and lack of sensitivity necessary
for diagnostic applications for all viruses (eg, not applicable for
rhinoviruses because there are >90 serotypes and cross- reacting
antibodies).
e techniques commonly used for antigen detection include
immunouorescence assay (IFA; direct and indirect), EIA
(including ELISA), chemiluminescent and uorescence- based
immunoassay, and particle agglutination assays. Several membrane immunochromatographic assays (dipstick tests) are available as inuenza diagnostic tests (eg, Directigen Flu A or A+B
Test, QuickVue inuenza).
become simple to use, are low cost, and allow rapid detection
(≤30 minutes) of specic antigens from a single specimen at
POC (eg, outpatient facilities, physician oces, patient bedside)
and in the clinical laboratory. Many of these RIDT kits are CLIAwaived because the methodologies are simple to use and accurate, and the likelihood of erroneous results is negligible.
Rapid inuenza diagnostic tests (RIDTs) have lower specicity
Examples of viruses that can be identied by direct
1,13,32,39-41
e advantages of direct antigen detection include
13,32,39,40
ese viral antigen tests have
13,39,45
but variable sensitivity (higher in children and for detecting
inuenza A). e need for improved sensitivity of RIDTs was
also observed during the 2009 pandemic of H1N1 inuenza.
ese issues, in part, resulted in a medical device reclassication
by the FDA in 2017 and additional compliance requirements
of RIDTs for inuenza.
13,39,45
Further details on RIDTs and NA
detection- based tests for inuenza virus can be found at the
CDC website (https//www.cdc.gov/u/professionals/diagnosis
/rapidlab.htm).
Serology
Serologic tests are designed to detect an antibody response in
serum samples aer exposure to viral antigens has occurred.32
e major uses of serology for the detection of viral infections
include the demonstration of immunity or exposure to a virus,
the diagnosis of postinfectious sequelae, and the screening of
blood products. In several clinical situations, serologic testing remains the primary means for the laboratory diagnosis of
viruses that are dicult to culture or detect by direct methods
(eg, rubella virus, Epstein-Barr virus, hepatitis viruses, HIV,
arboviruses).
ive or adjunctive role in clinical situations in which viral cultures
or direct detection methods are available.
For viral infections, serologic testing can identify the virus,
distinguish the strain or serotype, dierentiate between primary
infection and reinfection, and determine if the infection is in
an acute or convalescent phase. Virus- specic immunoglobulin
antibodies (eg, IgM or IgG) are produced during the time course
of a viral infection. In general, virus- specic IgM is detected in
serum sooner than virus- specic IgG. e results measure the
relative concentration of antibody in the body as a titer, with the
titer representing the lowest antibody concentration (or inverse
of the greatest dilution; a dilution of 1:128 is expressed as a titer
of 128) that demonstrates activity in a patient’s serum. e exact
value for a titer varies with each testing method, the specic
virus involved, the timing of specimen collection, and the presence of active disease.
For most viral infections, virus- specic IgM can be detected
as soon as 3 to 7 days aer the onset of infection. e presence
of virus- specic IgM in a single serum sample shortly aer the
onset of symptoms (acute phase) is usually indicative of a recent
or current primary infection. Titers of virus- specic IgM usually
decline to near undetectable amounts within 1 to 4 months aer
the onset of infection. Virus- specic IgG can be detected during the acute phase of infection (eg, 1 to 2 weeks) and continues
to increase for several months before reaching a maximal titer.
ereaer, the IgG titer declines, but it usually remains detectable in serum for the remainder of a person’s life. Seroconversion has occurred when at least a 4- fold increase in IgG titer has
occurred between serum samples collected in the acute and convalescent (two to four weeks aerward) phases. e presence of
virus- specic IgG is also indicative of a past infection.
Serologic tests are also used to assess the immunity or
exposure to a virus. e presence of antibody can detect
which patients have been previously infected by or vaccinated for a specic virus. For example, a positive result (presence of antibody) for rubella in a woman of childbearing age
1,13,32
Serologic testing may also serve as a support-
474 BASIC SKILLS IN INTERPRETING LABORATORY DATA
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implies that congenital infection will not occur during subsequent pregnancies. A negative result (absence of antibody)
implies susceptibility to infection, and the woman should
receive rubella vaccination as a preventative measure if she
is not pregnant. Some other examples of viruses for which
serologic determination of immune status is useful include
hepatitis A and B (HAV, HBV), measles, mumps, parvovirus
B19, and VZV.
1,13,32
e techniques commonly used for serologic assays include
CF, EIA, IFA, anticomplement immunouorescence, and western immunoblotting. In the diagnosis of certain viral syndromes
(eg, CNS infections), a serology panel may be helpful so that a
battery of antigens is tested for antibody to several viruses. e
advantages of viral serology include the assessment of immunity or response of a virus isolated from a nonsterile site, serum
specimens are easy to obtain and store, and it can be used to
identify viruses that are dicult to culture or detect by immunoassay. e disadvantages include the time to results (eg, few
days to weeks), the potential for cross- reactions between different viruses, and the need for both acute and convalescent
specimens.
Antiviral Susceptibility Testing
e emergence of drug- resistant strains of viruses to antiviral
agents is an increasing problem, especially in immunocompromised hosts. Unlike antibiotics, invitro susceptibility testing
of viruses has not been routinely available. e major variables
that have limited the standardization of antiviral susceptibility testing include cell lines, inoculums titer, incubation period,
testing range of antiviral drug concentrations, reference strains,
assay methodology, and criteria, calculation, and interpretation
of end points.46 However, emerging molecular technology and
the phasing out of virus culture- based methods have permitted
phenotypic and genotypic antiviral susceptibility testing vividly
advance.
e FDA- cleared assays for viral susceptibility testing for
the past decade have mainly been limited to phenotypic and
genotypic assays for HIV. Antiviral resistance and cases of drug
failure has led to increased interest in susceptibility testing of
HSV, VZV, CMV, and inuenza viruses.46 us far, most susceptibility testing for these pathogens has been limited to research
use only or laboratory user- developed tests. e CLSI has published an approved standard for phenotypic susceptibility testing
of HSV.47 is standard outlined the use of a plaque reduction
assay and denotes resistance to acyclovir and foscarnet when
inhibitory concentration 50% (IC50) values are ≥ 2 mcg/mL and
≥ 100 mcg/mL, respectively. Proposed guidelines for antiviral
susceptibility results of HSV, CMV, VZV, and inuenza A and B
viruses for various other phenotypic testing methods (ie, DNA
hybridization, EIA, neuraminidase inhibition assay, late antigen
reduction assay) and antiviral agents (ie, famciclovir, vidarabine, cidofovir, ganciclovir, neuraminidase inhibitors) have also
been outlined.46 Interpretation of these values must be carefully
made in conjunction with the clinical response of the individual
patient. Additional consensus documents and further standardization of phenotypic and genotypic assays for antiviral susceptibility testing are needed.
HUMAN IMMUNODEFICIENCY VIRUS
Human immunodeciency virus (HIV) is the causative agent of
AIDS. e HIV virus is an enveloped, positively stranded RNA
virus that belongs to the Retroviridae (retrovirus) family and
Lentivirus genus.48 e mature virus measures approximately
100nm in diameter and has a characteristic conical core containing proteins, enzymes, and two identical copies of single- stranded
RNA. Viral proteins within the core and the lipid envelope play
a signicant role in the detection, diagnosis, and treatment of
49,50
HIV.
e replication process of HIV involves transcription
of viral RNA into proviral DNA using the reverse transcriptase
(RT) enzyme. e proviral DNA is then integrated into the host’s
genome using the integrase enzyme, resulting in lifelong latent
infection. e virus is transmitted to humans by the exchange of
blood or other body uids containing the virus through sexual
contact; exposure to contaminated blood; transfusion of contaminated blood and blood products; or via contaminated needles
(eg, intravenous drug users or accidental needle sticks). In addition, infants can acquire HIV from an infected mother in utero,
during labor or delivery, or during breastfeeding.
49,51
ere are two distinct serotypes of HIV, namely HIV-1 and
HIV-2; while HIV-1 is the most prevalent serotype of HIV infections worldwide, HIV-2 infection is most commonly distributed
in Western Africa and other limited geographic locations.
49-51
Routine diagnostic testing of HIV-2 is not recommended in the
United States because its prevalence is extremely low. us, the
following discussion focuses mainly on laboratory tests used for
the diagnosis and management of HIV-1 infection. However,
HIV-2 testing may be indicated in persons at risk for HIV-2
infection or for those who have symptoms suggestive of HIV
infection with negative or indeterminate test results for HIV-1.
In addition, all blood donations in the United States are tested
for both HIV-1 and HIV-2.
49-51
Laboratory Tests for Human Immunodeficiency
Virus-1 Infection
Several laboratory tests are available for the diagnosis and monitoring of patients with HIV-1 infection. e most common virologic testing methods include HIV-1 antibody assays, HIV-1
p24 antigen assays, DNA-PCR, plasma HIV-1 RNA (viral load)
assays, and viral phenotypic and genotypic assays. In addition,
the absolute number of CD4+ lymphocytes and the ratio of
helper (CD4+) to suppressor (CD8+) lymphocytes (CD4+:CD8+
ratios) are routinely measured to evaluate the patient’s immune
status and response to antiretroviral therapy, because HIV primarily infects and depletes CD4+ T helper lymphocytes. Viral
cultures for HIV are not typically performed beyond clinical
research studies due to the labor- intensive nature of the testing methods as well as the extensive time required to obtain
33-35
results.
Laboratory tests for HIV-1 infection are clinically used for
diagnosing HIV-1 infection, monitoring progression of HIV
infection and the response to antiretroviral therapy, and screening blood donors. e selection of these tests is highly dependent on the clinical situation, the patient population, and the
specied purpose for the testing, as described in Table19-8.
8,51-54
CHAPTER 19 • InfECTIous DIsEAsEs: fungI, VIRusEs, AnD MyCobACTERIA 475
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TABLE 19-8.
CLINICAL SITUATION RECOMMENDED TEST(S) COMMENTS
<
antibody differentiation
immunoassay
+
differentiation immunoassay
differentiation immunoassay
further testing
testing is indeterminate
load
tests
+
cell count
cell count
T
>
+
T
Antiretroviral drug resistance testing
assays
<
Blood donor screening
+
Source
e following section briey reviews each of the specic tests,
but more comprehensive descriptions of the various commercial assays and their clinical applications can be found
elsewhere.
49,51,52,55
detected in the blood by 4 to 8 weeks aer exposure to the virus.
However, it may take up to 6 to 12 months in some patients.
ere are several tests currently available for the detection of
HIV antibody in infected patients.
e methodology of EIA (commonly referred to as ELISA)
Human Immunodeficiency Virus Antibody Tests
Infection with HIV aects both humoral and cell- mediated
immune function. e humoral immune response results in the
production of antibodies directed against HIV- specic proteins
and glycoproteins. For most patients, antibodies to HIV-1 can be
is widely used as the initial screening test to detect HIV- specic
antibodies.
49,51,52,56
Like all immunoassays, ELISA is based on the
concept of antigen and antibody reaction to form a measurable
precipitate. e ability of ELISA to detect HIV antibodies during earlier infection has improved over recent years. Although
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