ARTICLE: Streptococcal antibody tests (ASO, DB, Streptozyme)

[Guest guest]

http://findarticles.com/p/articles/mi_g2601/is_0013/ai_2601001303/

Streptococcal antibody tests

Encyclopedia of Medicine, Apr 06, 2001 by Janis O.

Streptococcal

infections are caused by a microorganism called Streptococcus. Three

streptococcal antibody tests are available: the antistreptolysin O

titer (ASO), the antideoxyribonuclease-B titer (anti-Dnase-B, or ADB),

and the streptozyme test.

The

antistreptolysin O titer, or ASO, is ordered primarily to determine

whether a previous group A Streptococcus infection has caused a

poststreptococcal disease, such as scarlet fever, rheumatic fever, or a

kidney disease called glomerulonephritis.

The

anti-DNase-B (ADB) test is performed to determine a previous infection

of a specific type of Streptococcus, group A beta-hemolytic

Streptococcus. Identification of infections of this type are

particularly important in suspected cases of acute rheumatic fever

(ARF) or acute glomerulonephritis.

Streptozyme

is a screening test used to detect antibodies to several streptococcal

antigens. An antigen is a substance that can trigger an immune

response, resulting in production of an antibody as part of the body's

defense against infection and disease.

For

the ASO test, increased levels of fats, called beta lipoproteins, in

the blood can neutralize streptolysin O and cause a false-positive ASO

titer. Antibiotics, which reduce the number of streptococci and thereby

suppress ASO production, may decrease ASO levels. Steroids, which

suppress the immune system, consequently may also suppress ASO

production. Also Group A streptococcal infections of the skin may not

produce an ASO response. Antibiotics also may decrease anti-DNase-B

(ADB) levels.

Streptococcal

infections are caused by bacteria known as Streptococcus. There are

several disease-causing strains of streptococci (groups A, B, C, D, and

G), which are identified by their behavior, chemistry, and appearance.

Each group causes specific types of infections and symptoms. These

antibody tests are useful for group A streptococci. Group A

streptococci are the most virulent species for humans and are the cause

of "strep throat," tonsillitis, wound and skin infections, blood

infections (septicemia), scarlet fever, pneumonia, rheumatic fever,

Sydenham's chorea (formerly called St. Vitus' dance), and

glomerulonephritis.

Although

symptoms may suggest a streptococcal infection, the diagnosis must be

confirmed by tests. The best procedure, and one that is used for an

acute infection, is to take a sample from the infected area for

culture, a means of growing bacteria artificially in the laboratory.

However, cultures are useless about two to three weeks after initial

infection, so the ASO, anti-DNase-B, and streptozyme tests are used to

determine if a streptococcal infection was present.

The ASO titer is used to demonstrate the body's reaction to an

infection caused by group A beta-hemolytic streptococci. Group A

streptococci produce the enzyme streptolysin O, which can destroy

(lyse) red blood cells. Because streptolysin O is antigenic (contains a

protein foreign to the body), the body reacts by producing

antistreptolysin O (ASO), which is a neutralizing antibody. ASO appears

in the blood serum one week to one month after the onset of a strep

infection. A high titer (high levels of ASO) is not specific for any

type of poststreptococcal disease, but it does indicate if a

streptococcal infection is or has been present.

Serial

(several given in a row) ASO testing is often performed to determine

the difference between an acute or convalescent blood sample. The

diagnosis of a previous strep infection is confirmed when serial titers

of ASO rise over a period of weeks, then fall slowly. ASO titers peak

during the third week after the onset of acute symptoms of a

streptococcal disease; at six months after onset, approximately 30% of

patients exhibit abnormal titers.

Anti-DNase-B,

or ADB, also detects antigens produced by group A strep, and is

elevated in most patients with rheumatic fever and poststreptococcal

glomerulonephritis. This test is often done concurrently with the ASO

titer, and subsequent testing is usually performed to detect

differences in the acute and convalescent blood samples. When ASO and

ADB are performed concurrently, 95% of previous strep infections are

detected. If both are repeatedly negative, the likelihood is that the

patient's symptoms are not caused by a poststreptococcal disease.

When

evaluating patients with acute rheumatic fever, the American Heart

Association recommends the ASO titer rather than ADB. Even though the

ADB is more sensitive than ASO, its results are too variable. It also

should be noted that, while ASO is the recommended test, when ASO and

ADB are done together, the combination is better than either ASO or ADB

alone.

The streptozyme

test is often used as a screening test for antibodies to the

streptococcal antigens NADase, DNase, streptokinase, streptolysin O,

and hyaluronidase. This test is most useful in evaluating suspected

poststreptococcal disease following Streptococcus pyogenes infection,

such as rheumatic fever.

Streptozyme

has certain advantages over ASO and ADB. It can detect several

antibodies in a single assay, it is technically quick and easy, and it

is unaffected by factors that can produce false-positives in the ASO

test. The disadvantages are that, while it detects different

antibodies, it does not determine which one has been detected, and it

is not as sensitive in children as in adults. In fact, borderline

antibody elevations, which could be significant in children, may not be

detected at all. As with the ASO and ADB, a serially rising titer is

more significant than a single determination.

Free 3D Earth Screensaver

Watch the Earth right on your desktop! Check it out at www.inbox.com/earth