Screening tests for group B strep infection

StreptococcusThe most common cause of life-threatening infections in newborns comes from a bacteria known as Streptococcus agalactiae (more commonly referred to as group B streptococcus or GBS).  This was stressed in a recent meta-analysis that reported that GBS infection remains an important, global cause of infant mortality.

The overall infection rate was 0.53 per 1,000 live births and, on average, about 10% of infected infants died.  Infants born in Africa were more likely to be infected (1.21 per 1,000) and die (22%) from the infection than infants born in the Americas or Europe (0.67-0.57 per 1,000 with 11 and 7% fatality rates).

It doesn't have to be this way because GBS infection is treatable with antibiotic therapy.  Indeed, in more developed countries, therapy is provided to women who carry the bacteria which prevents their baby becoming infected during delivery.  However, in poorer countries this is less likely to happen due to fewer resources.

Providing therapy to every pregnant women is not practical because not all women are colonized with GBS and so a key preventative strategy is to identify those women who do carry the bacteria.  The most sensitive test is culture performed on samples collected from the vagina and rectum.  The Centers for Disease Control and Prevention (CDC) published guidelines in 2010 that called for the routine GBS screening in all pregnant women at 35 to 37 weeks of gestation.  Testing needs to happen close to delivery (normally at ~40 weeks) because women can be colonized with GBS at anytime.  That is, a negative test result obtained earlier in pregnancy wouldn't rule-out the possibility that colonization then occured sometime after testing.  Women with a positive culture are treated with antibiotics during labor to prevent the transmission of GBS to their infant.

Although culture is considered the gold standard test for GBS screening, it is not perfect because some infants born to culture-negative women still get infected with GBS.  Also, culture techniques give results in 1–3 days, a time frame that may not be useful should an expectant mother go into labor prior to having the culture test performed.  For these women, DNA-based tests can be used.

These tests detect the presence of GBS using a DNA amplification technique like PCR and give results in a few hours rather than days.  Currently, these types of tests are not as sensitive as culture (i.e. they can give false-negative results) and so they aren't recommended for routine screening of women who are not in labor.  Their sensitivity is improved by using an enriched sample (one where the bacteria are allowed some time to multiply in a growth media), the use of this type of sample is impractical for women in labor when results are needed quickly.

Until an effective vaccine to prevent GBS infection is available, laboratory testing will remain an essential tool for identifying and preventing GBS.

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