Physicians are enamored of blood tests, as are patients. They seem so clear and unequivocal. But any good doctor knows that blood results do not a diagnosis make. Diagnosis is a complex, multivariate consideration. In the case of infection, serology can contribute significantly to making a diagnosis, but many factors go into whether serology is accurate or not.
In the case of Chlamydia pnemoniae, there are significant difficulties in obtaining accurate tests. In addition to the accuracy issue, individual's may not respond antigenically. All of this means that having a negative blood test for Cpn does not mean that you don't have Cpn infection. Tests can be inaccurate for the following, and other, reasons:
- The test may not be sensitive enough. Antigen tests for Cpn are said to be relatively inaccurate. PCR tests are the most sensitive, but dependent on the particular primers used, whether the dna used by the test matches the particular strain of Cpn one has, whether the test used accounts for the type of sample well (blood, sputum, cerebrospinal fluid, etc), the accuracy of the technician (PCR is particularly trickey in a technical sense), and so on.
- If the infection is predominantly in cryptic phase, there is no serology evident.
- If the patient's immune system is significantly depleted, they will not show positive antigens for the organism infecting them (they are immuno-incompetant).
Since many asymptomatic persons show existing antibodies to Cpn, and many with symptoms show no positive Cpn serology, blood tests are not the last word in diagnosis. Diagnosis requires a physician to put together the whole symptom picture: the suggestive evidence of infectious involvement, serology and other tests, to diagnose and treat a condition.