Rubella
Rubella (German measles) is a very common infection of childhood, although primary infection can occur in adults. The major clinical importance of rubella is maternal infection during pregnancy, which may produce the congenital rubella syndrome in the fetus. The congenital rubella syndrome includes one or more of the following: congenital heart disease, cataracts, deafness, and cerebral damage. Diagnosis is made by documenting active rubella infection in the mother during early pregnancy and by proving infection of the infant shortly after birth. Rubella antibody tests are used to determine (1) if a woman is susceptible to rubella infection (and, therefore, should be immunized to prevent infection during pregnancy), (2) to prove that a woman is immune (and therefore, does not have to be immunized or be concerned about rubella infection), (3) to determine if possible or actual exposure to rubella infection during pregnancy actually produced maternal infection, (4) to determine if an infant has been infected, (5) to determine if symptoms that might be rubella (such as a rash) really are due to rubella or to something else.
Fig. 17-1 Serologic tests in rubella infection.
Rubella has an incubation period of about 14 days (range, 10-23 days), followed by development of a skin rash that lasts about 3 days (range, 1-5 days). Illness can be subclinical in up to 25% of cases. The patients are contagious for about 2 weeks (range, 12-21 days), beginning 7 days (range, 5-7 days) before and ending about 7 days (range, 5-10 days) after onset of the rash. Subclinical illness is also infective. Virus can be cultured in the nasopharynx (posterior end of the nose is best) about 7 days before the rash until about 7 days (range, 4-15 days) after onset of the rash. Serologic tests have mostly replaced culture except for epidemiologic purposes.
Commercially available kits for antigen are not available. Those for antibody include hemagglutination inhibition (HI or HAI), indirect hemagglutination (IHA), ELISA, and LA. Most of the kits detect only IgG antibody, but some ELISA kits for IgM are also available. Some kits detect both IgM and IgG. Most current IgG kits appear to have greater than 95% sensitivity, although there is some variation between kits. There sometimes is confusion due to the large variety of kits and methods. Some kits detect both IgM and IgG, but do not differentiate between them and generally behave as though they detect IgG alone. Also, some procedures are reported as a titer and some as positive or negative. Also, HI (HAI) used to be the standard method but has been mostly replaced by ELISA and LA. Hemagglutination inhibition-reacting antibodies appear during the first week after onset of a rash; they are sometimes detectable after only 2-3 days. Peak levels are reached near the beginning of the second week after onset of the rash. Afterward the titer slowly falls, but an elevated titer persists for many years or for life. Although the standard HI test detects both IgM and IgG antibodies, the HI time sequence just described is similar to that of rubella IgG antibodies. Complement fixation-reacting or immunofluorescent-demonstrable antibodies develop in the more conventional time of 7-14 days after onset of the rash, reach a peak about 14-21 days after the rash and usually disappear in a few years.
Serologic tests for rubella IgM antibody are available. Immunoglobulin M antibody titer begins to rise about the time of onset of the rash, peaks about 1 week after onset of the rash, and becomes nondetectable about 4-5 weeks after onset of the rash (range 21 days-3 months). Therefore, the rubella IgM and IgG antibody rise and peak are relatively close together, in contrast to serologic behavior in most other viral diseases, in which IgG usually follows IgM by at least 1 week. Some IgM procedures, but not others, may be affected by IgM produced against nonrubella antigen (e.g., rheumatoid factor). If so, this might lead to a false positive result. Besides primary infection, rubella reinfection can occur. If this happens there is often a rise in IgG antibody, but IgM antibody is not produced. Reinfection of the mother during pregnancy is not dangerous to the fetus, in marked contrast to primary infection. The ELISA method generally detects about 94%-97% of nonneonatal patients with well-established rubella compared to the HI method and can be modified to detect either IgG or IgM or both together. Most LA kits detect over 95% of cases but detect only IgG.
Vaccination produces immune (IgM and IgG) response in about 95% of persons. Antibodies develop 10-28 days after vaccination. Some persons take up to 8 weeks to respond. Most of those who do not respond originally will do so if revaccinated. IgG elevation declines significantly in 10% of vaccinated persons by 5-8 years and becomes nondetectable in a small number of these persons (one study found about one third had no detectable IgG antibody at 10 years). IgM lasts longer than usual in vaccinated persons; in one study 72% still had detectable IgM at 6 months. Reinfection can occur, usually subclinical, more often in vaccinated persons than in those who had previous wild-type virus infection. Reinfection does not produce a detectable IgM response but may elevate the preexisting IgG level. Reinfection apparently does not harm a fetus.
When a test is reported either as positive or negative, this is a screen for immunity to rubella infection and is performed on a 1:8 serum dilution (the 1:8 dilution is the HI titer level that has become accepted as demonstrable of an immune IgG antibody response). If multiple serum dilutions are tested, the antibody responses detected by LA are similar in time sequence to the IgG response of HI.
Summary of Rubella Antibodies
HAI (HI): IS A TOTAL ANTIBODY TEST (IgM + IgG)
Appearance
1-3 days after onset of rash
Peak
About 14 days (range, 10-17 days) after onset of rash
Becomes nondetectable
Usually decreases about 2 serial dilutions by 1 year, then stable for life
Titer of 1:8 considered adequate immune level
IgM ANTIBODIES
Appearance
About 1-2 days after onset of rash
Peak
About 10 days (range, 7-21 days) after onset of rash
Becomes nondetectable
About 5-6 weeks (range, 10 days-12 weeks) after onset of rash; in congenital rubella, remains elevated after birth for 3-6 months
IgG ANTIBODIES
Appearance
About 3-4 days after onset of rash
Peak
About 14 days (range, 10-21 days) after onset or rash
Becomes nondetectable
Remains elevated for life
Absence of HI IgG (1:8 level) or LA antibody indicates susceptibility to rubella since elevated IgG levels usually persist for many years, whereas titers of other antibodies return to normal. Presence of LA antibody means either past or recent infection. In a person who is clinically well, this means immunity to subsequent infection. In a person with clinically suspected rubella, an immediate serum specimen and a second one drawn 2 weeks later should be obtained, the standard procedure for all serologic tests. A fourfold rise in titer confirms very recent (active) infection. However, if the first serum specimen was not obtained until several days after onset of a rash, the LA antibody titer peak may already have been reached, and no further increase may occur. If tests for rubella IgM antibody are available, presence of this antibody means recent acute infection. Absence of IgM antibody in a single specimen, however, does not completely rule out acute or recent infection, since the specimen could have been obtained either before antibody rise or after antibody fall. If IgM antibody tests are not available, a significant two-tube dilution or fourfold rise in titer of CF or fluorescent antibody may be demonstrable, since these antibodies develop later than LA. However, if both the LA and CF antibodies are at their peak, it is impossible with this information alone to differentiate between recent infection and infection occurring months or even years previously. Height of titers by itself is not reliable in differentiating acute from old infection; only a sufficient change in titer can provide this information.
Infants with congenital rubella infection can produce both IgM and IgG antibody before birth, beginning in the late second trimester. In addition, the fetus acquires passively transferred maternal IgG antibody, whether or not the mother acquired the infection during pregnancy, so that neonatal serum IgG antibodies could represent either old or current maternal infection. Therefore, neonatal serum IgG antibodies might originate from the infant, the mother, or both. By age 6-8 months, maternal antibody in the child has disappeared, and persistence of IgM or IgG antibody past this time indicates congenital or neonatal infection. For some reason, however, at least 20% of children with congenital rubella lose their HI titer by age 5 years. Congenital rubella can also be diagnosed by detection of specific rubella IgM antibody in the blood of the newborn. If the specimen is drawn before 10 days of life (the incubation period of rubella acquired during or just after birth before postnatal antibody has a chance to rise), specific rubella IgM antibody is diagnostic of intrauterine infection. If the specimen is obtained later, this antibody may be highly suggestive of congenital rubella but is not absolutely diagnostic, since there could be a small chance that infection was acquired after delivery.
The ELISA and LA tests are, in general, more reliable than the HI test in the average laboratory. However, false positive or negative results may occur for various reasons, just as they may occur with any test in any laboratory. If the patient is pregnant and test results may lead to some action, it may be advisable to split each sample, keeping part of each frozen, if the specimens are sent to an outside laboratory, in case a recheck is desired. If the tests are performed in-house, immediate redraw of a specimen that suggests active maternal infection might be useful. Because of technical factors, most laboratories list a specific titer below which the antibody level is not considered significant. This depends to some extent on the test being used. The cutoff titer level most frequently is 1:8 or 1:10. This fact is mentioned because theoretically any antibody titer ought to be significant in terms of indicating previous infection. However, in actual practice, antibody levels below the cutoff value are considered negative since it is not certain how much artifact is involved in very low titers.
Summary of Rubella Testing
For immune status = Single IgG antibody test
For primary acute infection diagnosis = IgM (if negative, repeat in 2 weeks) or IgG (using acute and convalescent specimens)
For congenital infection diagnosis = fetal/maternal IgM
For possible reinfection = IgG acute and convalescent (assuming IgG was known to have been elevated before the presumed reinfection occurred)
Summary of rubella test results
To test for immunity to rubella in a pregnant or nonpregnant woman, an LA test (or other standard rubella test) is obtained. If the result is negative, the woman is susceptible to infection. A positive test result means immunity; and in a nonpregnant woman and in many pregnant women, this is usually enough information. However, a positive test result could either be due to past infection or recent infection. If there is some reason to rule out recent infection in a pregnant or nonpregnant woman, a rubella IgM titer could be obtained. An alternative could be a titer of the original specimen plus another specimen for titer in 2 weeks. To determine whether recent infection took place, the time relationship of two critical events—date of exposure or date of rash—is extremely important regarding what test to use and when to obtain the test specimen or specimens. To determine the presence or absence of immunity only, such timing is not important.
If a pregnant woman has been exposed to someone with rubella, and the question is whether infection has been acquired, serum should be obtained immediately for rubella antibody titer. A significant titer obtained less than 10 days after exposure usually means immunity because of previous disease (the incubation period of rubella is 10-21 days). If the result is negative or a borderline low titer, a second specimen should be obtained 3-4 weeks later to detect a rising titer (to permit sufficient time for antibody to be produced if infection did occur). If exposure was more than 10 days previously and the LA titer is borderline or elevated, a second specimen should be obtained 2-3 weeks later to detect a possible rising titer. Alternatively, a rubella IgM antibody test could be obtained about 3 weeks after exposure. Significantly elevated IgM proves recent primary infection.
If a person develops a rash, and the question is whether it was due to rubella, two specimens for rubella antibody titer should be drawn, one immediately and the other 2 weeks later. Alternatively, a rubella IgM antibody test could be obtained 7 days after the rash onset.