The use of PCR in establishing causal relationships of microbial infections and clinical conditions
Please cite this article as follows: PCR Encyclopedia (2005) 1: 100345-33
PCR Encyclopedia (2005) 1: 100345-33
The use of PCR in establishing causal relationships of microbial infections and clinical conditions
Antonio Chuh, MD(HK) MRCP(UK) FRCP(Irel)
Part-time Associate Professor
Department of Community and Family Medicine
The Chinese University of Hong Kong
Honorary Clinical Assistant Professor
Department of Medicine
The University of Hong Kong
Email: achuh@iohk.com
PCR has extremely high sensitivities. Factors including cross-contamination and detection of sequences of host genomes render false positive results difficult to interpret. Viruses such as herpesviruses have lifelong latent infections with occasional opportunistic reactivations. Even in latent states these viruses can be detectable in clinical specimens. Such would lead to incorrect diagnoses and improper attributions that a particular disease is caused by a particular virus. The detection of these viruses in cell-free medium such as plasma and quantification of virus loads provide more valid clues for active infection. However, viraemia is not expected to be present in all disease states. Seroconversion, probably the gold standard criteria for primary viral infection, might not be easily documented unless the first specimen is collected from the patient at a very early phase of the condition. IgM, antibody avidity investigations, and significant increase in antibody titres are helpful in some circumstances but cross-reactivities render serology unreliable for some microbes.
Criteria such as Kochˇ¦s postulates (Rivers, 1937) and Hillˇ¦s criteria (Hill, 1965) are inappropriate to evaluate the strength of causal relationships detected by PCR as they do not take into account the aforementioned confounders. Guidelines (Fredericks and Relman, 1996) have been developed to differentiate a causal association from a confounding ˇ§innocent bystanderˇ¨ association. At the risk of oversimplification, the guidelines are:
1. Nucleic acid sequences of the microbe present in most cases of the disease, preferably in diseased organs or anatomical sites,
2. Few or no number of sequences present in hosts or non-diseased tissues,
3. Number of sequences decreasing with resolution, and increasing with relapse,
4. Sequence detection predating disease, or number of sequences correlating with disease severity,
5. Clinical features and pathologies consistent with biological characteristics of the microbe,
6. Tissue-sequence correlates detectable at cellular level, and
7. Sequence-based evidence be reproducible.
These guidelines are highly practical and have been adopted in systematic reviews to evaluate the strength of causal association between human herpesvirus-7 infection and pityriasis rosea (Chuh et al, 2004). Further work should be performed to establish the validity of such guidelines in application to a wider range of clinical conditions and to evaluate the limitations of such guidelines.
References
Chuh A, Chan H, Zawar V. Is human herpesvirus 7 infection the causative agent of pityriasis rosea? ˇV a critical review. Int J Dermatol 2004; 43: 870-5.
Fredericks DN, Relman DA. Sequence-based identification of microbial pathogens: a reconsideration of Koch's postulates. Clin Microbiol Rev 1996; 9: 18-33.
Hill AB. The environment and disease: association or causation? Proc Royal Soc Med 1965; 58: 295-300.
Rivers TM. Viruses and Kochˇ¦s postulates. J Bacteriol 1937; 33: 1-12.
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