Polymerase Chain Reaction (PCR) Technique in the Diagnosis of Gonococcal Urethritis
In clinical routine, the detection method for gonococcus etiology uses smear tests, and when necessary, gonococcus isolated culture is performed. However, this process takes a long time (3-4 days), cannot provide quick diagnosis, and has poor sensitivity and specificity with a low positive rate.
The test results showed that: PCR and smear positive rates were 92% and 56%, respectively. The PCR positive rate was significantly higher than that of the smear method. PCR technology performs direct rapid detection of gonococcus DNA from urethral secretion specimens at the molecular biology level, with high sensitivity and specificity. Using the PCR method, specific DNA fragments (CppB gene) of the gonococcus cryptic plasmid are amplified in vitro. Within a short period, these specific DNA fragments undergo exponential amplification instead of an increase in bacterial numbers [3]. Due to the high specificity of the CppB gene and its exponential amplification in a short time, PCR testing for gonorrhea exhibits high specificity and sensitivity, allowing for quick detection that can be completed within 5 hours. Compared to clinical smear and gonococcus isolated etiological diagnosis, PCR offers the following advantages: (1) fast testing speed; (2) advanced, reliable, and highly specific; (3) high sensitivity with a low misdiagnosis rate; (4) high positive rate, as long as there is a complete specific gene fragment, it can also be amplified for diagnosis [4].
A positive PCR test, combined with clinical findings, can lead to a definitive diagnosis, but false positives and false negatives may also occur. In this group of 179 patients, 26 patients tested positive by the smear method but negative by PCR. Among them, 9 were clinically cured patients who, after a period of drug withdrawal, tested negative by the smear method but still tested positive by PCR. While the PCR method has certain advantages over the smear method in diagnosis, it also has limitations. Therefore, the possibility of PCR missing a diagnosis cannot be ignored because: (1) specimens may contain impurities that inhibit Tag polymerase activity; (2) specimen containers should be clean to prevent contamination by PCR inhibitors; (3) changes in the pH value of the reaction system can affect PCR amplification; (4) the selection of primers must have homology with all known sequences of gonococcus to be measured, aiming for high specificity and repetitive sequences as target sequences to achieve the highest yield; (5) if the amount of gonococcus in the specimen is too small or lacks a complete specific gene fragment, it can affect amplification for diagnosis [5-7].