How do you diagnose rubella in a pregnant woman with suspected infection?

Diagnosis of Rubella in Pregnancy

Diagnose rubella in pregnant women through serologic confirmation using rubella-specific IgM antibody testing or demonstration of a significant rise in IgG antibody levels between acute and convalescent sera, with timing of specimen collection being critical for accurate results. 1

Serologic Confirmation Methods

Primary Diagnostic Approach: IgM Antibody Testing

• Rubella-specific IgM antibody is the most direct evidence of acute infection and should be the first-line diagnostic test. 1, 2
• Draw the serum specimen between 1-2 weeks after rash onset for optimal IgM detection. 1
• IgM antibody is less likely to be detected if the specimen is drawn earlier than 1 week or later than 4-5 weeks following rash onset. 1
• Critical caveat: False-positive IgM results can occur, including after reinfection or other viral infections, requiring cautious interpretation. 3
• False-negative IgM results may also occur even when specimens are appropriately timed. 1

Alternative Approach: Paired Sera Testing

• Demonstrate a fourfold rise in IgG antibody titer between acute and convalescent specimens. 1
• Draw the acute-phase specimen as soon after rash onset as possible, preferably within the first 7 days. 1
• Draw the convalescent-phase specimen 10 or more days after the acute-phase specimen. 1
• Test both specimens simultaneously in the same laboratory to ensure accurate comparison. 1

Special Considerations for Pregnant Women

Timing Challenges

• If the acute-phase specimen is drawn more than 7 days after rash onset, a significant antibody rise may not be detected by most commonly used tests. 1
• In this scenario, complement fixation (CF) testing may be especially useful because CF antibodies appear later than HI, EIA, or IFA antibodies. 1

Exposure Without Rash (Subclinical Cases)

• Obtain the acute-phase serum specimen as soon as possible after exposure. 1
• Draw the convalescent-phase specimen 28 or more days after exposure. 1
• If paired sera provide inconclusive results, perform rubella-specific IgM antibody testing, but interpret results cautiously and consider expert consultation. 1

Critical Diagnostic Pitfalls

The Challenge in Pregnant Women

• Confirmation of rubella infection in pregnant women of unknown immune status following rash illness or exposure is often difficult, and serologic results are frequently nonconfirmatory. 1
• Obtain a serum specimen as soon as possible when infection is suspected. 1

Prevention of Diagnostic Difficulties

• Perform prenatal serologies routinely to establish baseline immune status. 1
• Request that laboratories performing prenatal screening retain specimens until delivery so that retesting can be done if necessary. 1

Advanced Diagnostic Options

Fetal Blood Sampling

• Detection of rubella-specific IgM antibodies in fetal blood can establish prenatal diagnosis of fetal infection. 4, 3
• Fetal blood sampling is typically performed between 21-23 weeks of pregnancy. 5
• Very sensitive techniques must be used for reliable results, and false-negative results can occur. 3, 5

Molecular Testing

• Detection of rubella virus RNA by reverse transcriptase-polymerase chain reaction (RT-PCR) can confirm infection. 2

Clinical Context for Diagnosis

When to Suspect Rubella

• Look for fever and lymphadenopathy followed by an erythematous, maculopapular rash. 2
• The rash classically begins on the face, spreads cephalocaudally, becomes generalized within 24 hours, and disappears within 3 days. 2
• In adults, joint symptoms (arthralgia/arthritis) occur in up to 70% of infected women. 6

Gestational Age and Risk Assessment

• The risk of congenital defects is up to 85% when infection occurs in the first 8 weeks of gestation. 6, 4
• Risk declines to 10-24% when infection occurs between 13-16 weeks. 6
• Defects are rarely noted when infection occurs after the 16th week of gestation. 4