Accuracy, sensitivity and specificity analysis

It can be affected by factors such as: The main outcome of a diagnostic imaging examination is an accurate diagnosis of a patients condition so that proper treatment can be administered. It can be affected by factors such as: Image quality Competency of the radiologist to interpret the image

Classification of a Diagnosis Positive – patients in whom the disease has been diagnosed. True positive – if the diagnosis is revealed to be correct. False positive – if study indicates that they do not have the disease despite the positive findings from the image. Negative – healthy patients with no disease present are considered negative.

True negative – if a diagnosis of a negative is determined from an image and is supported by follow-up studies. False negative – if a negative diagnosis is given to a patient who later has the disease.

Accuracy Is the percentage of cases that is diagnosed correctly; it can be determined by the following equation: Accuracy = (NTp + NTn)/Ntotal =100 N = number of cases NTp = Number of cases of True Positive NTn = Number of cases of True Negative Ntotal = total number of cases

Example If a 210 total chest x-ray are performed in a month and the number of true negatives is 167 and the number of true positive is 36, then the accuracy rate of the image diagnosis is Answer: 0.967 or 96.7%

Sensitivity Is also referred to as the TP fraction (True Positive Fraction) and indicates the likelihood of obtaining a positive diagnosis in a patient with the disease. Sensitivity is determined by the following equation. Sensitivity = NTp/(NTp + NFn) NFn – False Negative

Example If a department has a true positive of 36 and has a false negative of 3, what is the sensitivity of the diagnosis. Answer: 0.92 or 92%

Specificity Is also known as the TN fraction (True Negative Fraction) and indicates the likelihood of a patient obtaining a negative diagnosis when no disease is present. Specificity is determined by the following equation Specificity = NTn/(NTn + NFp) NFp = False positive

Example A department receiving a 167 TNs and 4 FPs has a specificity of Answer: 0.97 or 97%

Problem solving If a department has a total number of 300 cases , with a true positive of 157 and a true negative of 115 with a false negative of 5 and a false positive of 2. What is the accuracy percentage? What is the sensitivity percentage? What is the specificity percentage?

Answer Accuracy = 157 + 115/300 x 100 = 0.906 or 90.6% Sensitivity = 157/157 + 5 = 0.969 or 96.9% Specificity = 115/115 + 2 = 0.982 or 98.2%

Practice If a department has a causal rate of 12% that was caused by the processor what is the number of films used if it has a total repeat rate of 432? x/432 = 0.12 432*0.12 = x X=51.84 or 52 films

Data analysis With these data shows that the department has an accuracy 90.6% to which cases of diagnosis is correct, sensitivity or detectability of 96.9% which is the likelihood of obtaining a positive diagnosis of patients with a disease. A specificity of 98.2% to which it indicates the likelihood of a patient obtaining a negative diagnosis when no disease is present.

Summary The ideal for all of the values is 100%. A diagnostic imaging department is responsible for establishing its own threshold of acceptability for each value. Implementing a quality management program requires more than just equipment monitoring and maintenance. The outcome assessment of diagnostic images must also be performed to evaluate the success of the procedure.

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