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Subject: Cytomegalovirus (CMV) Culture (Rule Out)
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CMV is a ubiquitous viral pathogen. Most infections in immunocompetent patients are asymptomatic or mildly symptomatic, including a self-limited mononucleosis syndrome. In immunocompromised patient populations, including neonates, patients with AIDS, and transplant patients, serious localized (e.g., retinitis, colitis, polyradiculopathy, encephalopathy) or systemic infection may occur.
Specimens for CMV culture are usually inoculated onto monolayers of human fibroblasts (e.g., foreskin, fetal lung). Tube cultures should always be inoculated for CMV cultures. The shell vial cultures may also be inoculated. Shell vial cultures provide a more rapid turnaround time than tube cultures but are somewhat less sensitive for detection.
Presumptive CMV infection may be inferred by typical cytopathic effect, but positive cultures should be confirmed by immunologic techniques, such as DFA staining with CMV-specific reagents.
Turnaround time: Specimens with high viral loads, such as urine, may give positive results within several days, but negative cultures may require incubation for up to 4 weeks before signing out as negative. Shell vial cultures are processed for growth at 48–72 hours after inoculation.
Specimens should be collected according to general recommendations for virus culture of the specimen type.
Specimens should be collected early in acute infection.
Urine is most often recommended for evaluation of neonates with suspected CMV infection. For evaluation of patients with suspected viremia, heparinized whole blood or isolated buffy coat cells are used to inoculate cultures.
CMV is a fastidious virus and should be delivered to the laboratory as quickly as possible. Most specimens should be placed in a viral transport medium and transported at 4°C; do not freeze.
Expected results: Negative.
Negative results: Negative cultures do not rule out CMV infection; they may be due to loss of viability after collection or low viral load in the specimen submitted.
Positive results: Positive cultures usually indicate active CMV infection. Occasionally, positive cultures represent asymptomatic shedding of virus not associated with disease.
Positive cultures may be due to asymptomatic shedding during latent infection; correlation with histopathology, and other clinical signs and symptoms may be needed to ensure specific diagnosis.