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Clinical Laboratory Medicine Book

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This thoroughly updated Second Edition of Clinical Laboratory Medicine provides the most complete, current, and clinically oriented information in the field. Clinical Laboratory Medicine: Clinical Application of Laboratory Data. Front Cover. Richard Ravel. Year Book Medical Publishers, - Medical - pages. Clinical Laboratory Medicine. Front Cover. Kenneth D. McClatchey. Williams & Wilkins, - Diagnosis, Laboratory - pages. 0 Reviews.

This allows laboratory analyzers, computers and staff to recognize what tests are pending, and also gives a location such as a hospital department, doctor or other customer for results reporting. Once the specimens are assigned a laboratory number by the LIS, a sticker is typically printed that can be placed on the tubes or specimen containers.

This label has a barcode that can be scanned by automated analyzers and test requests uploaded to the analyzer from the LIS. Specimens are prepared for analysis in various ways.

For example, chemistry samples are usually centrifuged and the serum or plasma is separated and tested. If the specimen needs to go on more than one analyzer, it can be divided into separate tubes. Many specimens end up in one or more sophisticated automated analysers , that process a fraction of the sample to return one or more test results.

Some laboratories use robotic sample handlers Laboratory automation to optimize the workflow and reduce the risk of contamination from sample handling by the staff. The work flow in a hospital laboratory is usually heaviest from am to am. Nurses and doctors generally have their patients tested at least once a day with common tests such as complete blood counts and chemistry profiles.

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These orders are typically drawn during a morning run by phlebotomists for results to be available in the patient's charts for the attending physicians to consult during their morning rounds.

Another busy time for the lab is after pm when private practice physician offices are closing. Couriers will pick up specimens that have been drawn throughout the day and deliver them to the lab. Also, couriers will stop at outpatient drawing centers and pick up specimens. These specimens will be processed in the evening and overnight to ensure results will be available the following day. Laboratory informatics[ edit ] The large amount of information processed in laboratories is managed by a system of software programs, computers, and terminology standards that exchange data about patients, test requests, and test results known as a Laboratory information system or LIS.

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These systems enable hospitals and labs to order the correct test requests for each patient, keep track of individual patient and specimen histories, and help guarantee a better quality of results. Results are made available to care providers electronically or by printed hard copies for patient charts. Mayo Communique. Algorithm for identifying individuals with thyroid disorders based on TSH level. Ordering the following 20 laboratory and other tests on a 4-year-old child with signs and symptoms of an upper respiratory tract infection, fever Because the clinical performance characteristics of all laboratory tests differ with respect to their diagnostic accuracy ie, sensitivity and specificity , the selection of the appropriate laboratory test to order will vary depending on the purpose for which the test is to be used.

Before considering this aspect of the selection of laboratory tests, we must first understand the terms that describe their diagnostic performance. These terms include prevalence, sensitivity, specificity, efficiency, and predictive value. To illustrate the mathematical calculation of values for each of these parameters, consider the example given below: Example 2 The laboratory test, prostate-specific antigen PSA , was studied with regard to its ability to discriminate patients with prostate cancer PCa from those without PCa.

This test was performed on 10, men, of whom have biopsy-proven prostate cancer. Using this information, a 2 x 2 table can be constructed as shown below:. From this data, the values for prevalence, sensitivity, specificity, efficiency, positive predictive value PPV , and negative predictive value NPV can be determined:.

Most clinicians can achieve this total with a good history and physical examination! The poor PPV 2. If the test is used selectively, however, for example on a population of individuals likely to have a disease eg, a population in which the prevalence of disease is high , many laboratory tests have excellent PPVs.

The effect of prevalence on predictive value is demonstrated in Table 2. How do physicians increase the predictive value of laboratory tests? By appropriately selecting patients on whom the test is performed ie, by maximizing the prevalence of disease in the population sampled. For example, if the risk associated with failure to diagnose a particular disease is high eg, acquired immunodeficiency syndrome [AIDS] , false-negatives are unacceptable and only a laboratory test with high sensitivity is acceptable.

On the other hand, if a disease is potentially fatal and no therapy, other than supportive care, is available eg, cystic fibrosis , false-positives would be unacceptable. Thus, in this situation, a laboratory test with high specificity is desirable.

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In general, laboratory tests with both high sensitivity and high specificity are desirable since both false-negatives and false-positives are equally unacceptable under most clinical circumstances. Diagnostic sensitivity refers to the proportion of individuals with disease who yield a positive test for an analyte eg, PSA associated with a particular disease. Diagnostic specificity refers to the proportion of individuals without disease who yield a negative test for the analyte.

For any given test, there is always a trade-off between sensitivity and specificity, such that choosing a cutoff value decision threshold for a particular test that maximizes sensitivity occurs at the expense of specificity. This situation is illustrated in Figure 2. Dramatic representation of diagnostic sensitivity and specificity using the analyte prostate-specific antigen PSA as an example.

Visual inspection of Figure 2. Alternatively, if the cutoff value is increased to Lastly, it is important to remember that knowing the sensitivity ie, positivity in disease and specificity ie, negativity in health or non-disease of a test is of limited value because these parameters represent the answer to the question: What is the probability of a patient having a positive test result if this patient has disease X? The more challenging question facing clinicians, however, is: What is the probability of this patient having disease X if the test result is positive or negative?

Receiver- or relative- operator characteristic ROC curves provide another useful tool in assessing the diagnostic accuracy of a laboratory test, because all specificity, sensitivity pairs for a test are plotted.

The principal advantage of ROC curves is their ability to provide information on test performance at all decision thresholds.

The clinical usefulness or practical value of the information provided by ROC curves in patient care may vary, however, even for tests that have good discriminating ability ie, high sensitivity and specificity at a particular decision threshold. This may occur for several reasons:. False-negative results may be so costly that there is no cutoff value for the test that provides acceptable sensitivity and specificity.

B Prostatic acid phosphatase PAP and PSA in differentiating prostate cancer from benign prostatic hyperplasia and prostatitis at various cutoff values indicated adjacent to points on each of the curves.

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A more precise quantitative estimate of the superiority of one test over another can be obtained by comparing the area-under-the-curve AUC for each test and applying statistics to determine the significance of the difference between AUC values.

The AUC range: However, other data using ROC curves to assess the ability of the tumor markers, prostatic acid phosphatase PAP and prostate specific antigen PSA , to differentiate prostate cancer from BPH and prostatitis at various cutoff values is illustrated in Figure 2.

Once a clinical laboratory test with the appropriate diagnostic accuracy has been ordered, how are the results of the test interpreted? Decision level refers to a particular cutoff value for an analyte or test that enables individuals with a disorder or disease to be distinguished from those without the disorder or disease.

Moreover, if the diagnostic accuracy of the test and the prevalence of the disease in a reference population are known, then the predictive value of the decision level for the disorder or disease can be determined.

Reference interval relates to the values for an analyte eg, PSA, glucose, etc. All quantitative assays have a finite lower limit of detection LLD , distinct from 0, that more precisely constitutes the lower limit of the reference interval when this lower limit encompasses 0.

Therefore, any PSA value less than 0. In addition, it is important to remember that reference intervals for an analyte are method dependent ie, the reference interval established using one method cannot automatically be substituted for that of a different assay that measures the same analyte. Thus, reference intervals are intended to serve as a guideline for evaluating individual values and, for many analytes, information on the limits of an analyte for a population of individuals with the disease or diseases the test was designed to detect is even more informative.

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Related Items. Karen Spangenberg rated it really liked it Dec 13, Medical Laboratory Departments in some countries are exclusively directed by a specialized pathologist. Entry of test requests onto the LIS system involves typing or scanning where barcodes are used in the laboratory number, and entering the patient identification, as well as any tests requested.

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