The most useful laboratory classification of bacteria involves a threefold distinction: the Gram stain characteristics (gram positive or gram negative), morphology (coccus or bacillus), and oxygen requirements for growth (aerobic or anaerobic). Species exist that are morphologic exceptions, such as spirochetes; others are intermediate in oxygen requirements; still others are identified by other techniques, such as the acid-fast stain. Reaction to Gram stain has long been correlated with bacterial sensitivity to certain classes of antibiotics. A classic example is the susceptibility of most gram-positive organisms to penicillin. Morphology, when used in conjunction with this primary reaction, greatly simplifies identification of large bacterial groups, and oxygen growth requirements narrow the possibilities still further. The interrelationship of these characteristics also helps to control laboratory error. For example, if cocci seem to be gram negative instead of gram positive, a laboratory recheck of the decolorization step in the Gram procedure is called for since nearly all cocci are gram positive. If the staining technique is verified, the possibility of a small bacillus (Coccobacillus) or a Diplococcus must be considered.
Tag: laboratory
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Sensitivity and Specificity
Interpretation of laboratory test results is much more complicated than simply comparing the test result against a so-called normal range, labeling the test values normal or abnormal according to the normal range limits, and then fitting the result into patterns that indicate certain diseases. Certain basic considerations underlie interpretation of any test result and often are crucial when one decides whether a diagnosis can be made with reasonable certainty or whether a laboratory value should alter therapy.
All laboratory tests have certain attributes. Sensitivity refers to the ability of the test to detect patients with some specific disease (i.e., how often false negative results are encountered). A test sensitivity of 90% for disease Z indicates that in 10% of patients with disease Z, thetest will not detect the disease. Specificity describes how well test abnormality is restricted to those persons who have the disease in question (i.e., how often false positive results are produced). A specificity of 90% for disease Z indicates that 10% of test results suggestive of disease Z will, in fact, not be due to disease Z.