Positive predictive value

Histamine concentrations are maximal almost immediately, decrease thereafter with a half-life of about 20 min, and should be assayed within the first hour of a reaction. The sensitivity of this test for the diagnosis of anaphylaxis was estimated at 75%, the specificity at 51%, the positive predictive value at 75% and the negative predictive value at 51%. Tryptase reaches a peak in the patient s serum 30 min after the first clinical manifestations. Its half-life is 90 min, and the levels usually decrease over time. In a recent series, the sensitivity was estimated at 64%, specificity at 89.3%, positive predictive value at 92-95%, and negative predictive value at 54.3% [9]. 

In the discussion that follows positive predictive value (PPV) is defined as the proportion of people with a positive test result who have or will develop an unwanted response when the drug is administered. Negative predictive value (NPV) is the proportion of people with a negative test who will not have or develop an unwanted response when the drug is administered. 

Populations, gene flow between, 55-56 Population structure, 55-56 Positional cloning, 35, 36 Positive drug response, 262 Positive predictive value (PPV), 169-171 Postmarketing surveillance, 94—95, 324-325... 

Most researchers, even those who can deal with sensitivity, specificity, and predictive values, throw in the towel when it comes to Bayes theorem. This is odd, because a close look at the equation reveals that Bayes theorem is merely the formula for the positive predictive value (Box and Tiao, 1973). 

EXAMPLE 22.3. Use of Bayes Theorem or a 2x2 Table to Determine the Positive Predictive Value of a Hypothetical Tuberculin Screening Program... 

Suppose a pathologist is uncertain about an animal s cause of death and obtains a positive test result for a certain pathology. Even if the pathologist knows the sensitivity and specificity of the test, that does not solve the problem, because to calculate the positive predictive value, it is necessary to know the prevalence of the particular tissue or effect that the test is designed to detect. The prevalence is thought of as the expected prevalence in the population from which the animal comes. The actual prevalence is usually not known, but often a reasonable estimate can be made. 

Why did the posterior probability increase so much the second time One reason was that the prior probability was considerably higher in the second calculation than in the first (27% versus 2%), based on the fact that the first test yielded positive results. Another reason was that the specificity of the second test was quite high (98%), which markedly reduced the false-positive error rate and therefore increased the positive predictive value. 

Patients and blood donors are routinely screened for exposure to HIV by means of ElISA and Western blot assays of blood samples (F uie 1-7-15). The assays are designed to detect antibodies to HIV in the blood of the test subject The ELISA is used as the primary screening assay because it is very sensitive. Because the reference interval for the test is set to include everyone with antibodies to HIV, it also gives false positives and thus has a rather low positive predictive value, especially in low-risk populations. The Western blot (or immunoblot) is used as the confirmatory test for HIV exposure. In the Western blot technique, specific HIV proteins are separated by gel electrophoresis and blotted to a filter. The filter is incubated with the test sample. If the sample contains antibodies to HIV, they will bind to the proteins on the filter. The filter is next washed and incubated with a labeled goat anti-human IgG to visualize any bound human antibodies. The Western blot is highly specific. The combination of an ELISA and Western blot has a positive predictive value of greater than 99%,... 

Minutti CZ, Lacey JM, Magera MJ, Hahn, SH, McCann M, Schulze A, Cheillan D, Dorche C, Chace DH, Lymp JF, Zimmerman D, Rinaldo P, Matern D (2004) Steroid profiling by tandem mass spectrometry improves the positive predictive value of newborn screening for congenital adrenal hyperplasia J Clin Endocrinol Metab 89 3687-3693... 

NPV = negative predictive value PPV = positive predictive value ACS = acute coronary syndrome PTCA = percutaneous transluminal coronary angioplasty PCI = percutaneous coronary intervention DCCV = direct-current cardioversion A fib = atrial fibrillation NA = not applicable. 

Of the numerous questions that remain, the most prominent is how clinicians will interpret a positive IMA finding. The positive predictive value of the ACB test seems to be too low for use in ruling in ischemia, a use that clinicians hope the laboratory could provide. It is not known whether patients with negative ECG and necrosis markers (cardiac troponin) and a... 

Fig. 3. Predictive performance for active peptides by an ANN model. The positive predictive value (PPV, number of true positives/(true positives + false positives)) is shown versus the rate of positive predictions ((true positives + false positives)/(all predictions)) for varying ANN output threshold. ANN outputs above the threshold are positive predictions (i.e., predict peptide is active). For example, the threshold value of 0.5 on the curve corresponds to about 30% of the peptides are predicted to be active (positive) from the position on the x-axis, and the PPV is 0.27 from the y-axis. For the threshold value of 0.999, the PPV is 0.90, but only 2% of peptides are predicted active.

The importance of clinical question to the physician concerns the likelihood that the patient has food allergy if the test result is positive (positive predictive value). 

A negative prick test usually rules out food allergy mediated by IgE mechanisms. The PST with a commercial food allergen extract has a high negative predictive value (>95%), whereas a positive PST has only a 30%-40% positive predictive value (Burks and Sampson, 1992 Sampson, 1999a Nowak-Wegrzyn and Sampson,... 

Note PPV, positive predictive value klU/L is a unit for measurement of the concentration of food-specific IgE. 

Patch tests were positive in 89% of children with delayed-onset reactions, with challenge-proven milk allergy. In children with atopic dermatitis, the combination of positive patch tests with evidence of slgE or positive PST had the highest positive predictive value (see Table 3.3.7) (Roehr et al., 2001). 

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