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False negative results occur when elements present in the urine interfere with either the enzymatic reaction or prevent the oxidation of potassium iodide. Examples of such substances include: large quantities of ketones aspirin ascorbic acid > 50 mg/dL with some reagent strips levadopa 5-hydroxyindoleacetic acid homogentisic acid sodium fluoride ( a preservative)A specific gravity higher than 1.020 may lower glucose reagent sensitivity, especially in the presence of a high urine pH. Exposing reagent strips to excess humidity may also reduce glucose reagent reactivity.Check the package insert of the reagent strips used in your laboratory for interfering substances that may affect glucose results.

This test is sensitive to 0.06-0.1 mg/dL nitrite ion in urines with a low specific gravity and ascorbic acid concentrations of less than 25 mg/dL. Pink spots or pink edges should not be interpreted as a positive result because some medications can color urine red or turn red in an acid environment. Any degree of uniform pink color should be considered positive, suggesting the presence of 105 organisms/mL. Detection of low levels of nitrite ion may be enhanced by comparing the activated test strip to a white background. It is important to note that color development is NOT proportional to the number of bacteria present. The test is specific for nitrites and does not react with any other substances normally present in urine. Negative results do not necessarily rule out a urinary tract infection because yeasts or gram-positive bacteria unable to reduce nitrites may be the causative agent.

Since detectable amounts of bilirubin are not normally present in urine, results of the Ictotest® are reported as “positive” or “negative”, there is no quantitation. The sensitivity of Ictotest® is better than dipstick methods or the Harrison test. Ictotest® will detect as little as 0.05-0.10 mg bilirubin/dl urine, making it the procedure of choice for confirming bilirubin in urine specimens.

The presence of D-dimers in plasma or whole blood indicates that fibrin has been formed and degraded (fibrinolysis). Plasmin can also degrade intact fibrinogen, generating fibrinogen degradation products that are detected in fibrin/fibrinogen degradation products (FDP) assays. D-dimers and FDP can become elevated whenever the coagulation and fibrinolytic systems are activated. The presence of D-dimer confirms that both thrombin and plasmin have been generated since it can only be produced as the result of the plasmin degradation of fibrin. This makes the test for D-dimers more specific for fibrinolysis than the FDP test that also detects the products of the direct proteolysis of fibrinogen (fibrinogenolysis).The D-dimer test can be useful in the diagnosis of deep venous thrombosis (DVT) or pulmonary embolism (PE), two forms of venous thromboembolism (VTE). When the test is being used for this purpose, it is important that D-dimer levels are accurately measured and accurately reported because of the serious nature of this clinical decision. If the test is positive in a patient suspected to have DVT or PE, clinicians proceed with further diagnostic tests. If the test is negative, depending on the clinical situation and the sensitivity of the D-dimer assay, DVT or PE is considered unlikely and further diagnostic tests for DVT or PE might not be pursued. D-dimer is a sensitive, but not specific, diagnostic test for disseminated intravascular coagulation, and an indicator of increased risk of future myocardial infarction in patients evaluated for chest pain.

Molecular methodologies offer numerous advantages to the clinical laboratory. These include:Sensitivity: Amplification methodologies are particularly useful in increasing the sensitivity of a methodology and useful in the identification of target molecules of interest that are only present in low concentrations. Specificity: Molecular methods minimize false positive test results by targeting the specific molecule of interest.Turn Around Time: In comparison with standard traditional culture methods, molecular methodologies usually offer better turn around times from receipt to result reporting.Application: broader application can be found with molecular methodologies such as infectious diseases, genetic testing, forensics, drug resistance, and tumor marker detection and monitoring.

The product of an amplification reaction is termed an amplicon. Amplicons are the outcome of what is to be detected. Due to the sensitivity of amplification methodologies, and the nature of the amplicon, just a small amount of contamination can cause false positive results.Quality control of these various steps in the analytical procedure assure minimal contamination and quality results.

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Let's return to our experiment method from the previous example. This time, we'll calculate the sensitivity.The experimental method produced 600 true positives and 25 false negatives. By inserting these numbers into the sensitivity equation, we get (600 + (600 + 25)) x 100.Thus, the experimental method has a sensitivity of 96%.

To review, specificity is “disease focused”. The more specific a test is, the fewer false positive results will occur. Remember that a false positive result can possibly lead to a misdiagnosis with the possible consequence of unnecessary diagnostic procedures and therapies. Sensitivity, on the other hand, is “wellness or normal focused”. The more sensitive a test is, the fewer false negative results it produces.