cells that count. the standardizing of diagnostic tests for bovine mastitis bert nederbragt
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Cells that count. The standardizing of diagnostic tests for bovine mastitis Bert Nederbragt Descartes Centre for the History - PowerPoint PPT PresentationTRANSCRIPT
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Cells that count. The standardizing of Cells that count. The standardizing of diagnostic tests for bovine mastitisdiagnostic tests for bovine mastitis
Bert Nederbragt
Descartes Centre for the History and Philosophy of the Sciences and the Humanities and
Faculty of Veterinary Medicine
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epidemiology clinical trial
diagnosis
therapy development
pathogenesis
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epistemology (and social framing)
of a diagnostic test
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accuracy of the test defined as sensitivity and specificity
gold standard
a reference test that is supposed to determine a target condition or disease state unambiguously
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sensitivity = probability of a positive test
among patients with disease
specificity = probability of a negative test
among individuals without disease
sensitivity and specificity are never 1.0 false negatives and false positives may
occur
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consider alleged disease state as hypothesis/theory
consider test result as evidence for the theory
because of a probability of false positives or false negatives
underdetermination
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new test: gold standard as reference test
sensitivity and specificity of new test will always be less than those of the GS, although it may be a better test
undercalibration
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new test: cut-off point
below what point do we consider individuals normal, above which point ill or affected?
changing cut-off points changes specificity and sensitivity of the test specificity and sensitivity undercalibrated
>> cut-off point not decisive
underdiscrimination
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epistemology of the diagnostic test
bovine mastitis
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epistemology of the diagnostic test
decreased milk quality
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bovine mastitis:
inflammation of the udder by bacteria
clinical mastitis
redness, pain, nodes in tissue diagnosis: palpation
subclinical mastitis
predicts clinical mastitis decreased milkproduction
diagnosis: somatic cell count (SCC)
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somatic cell count gold standard
for subclinical mastitis
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what to do in case of underdetermination? (i.e. that testresults may be false positives or negatives)
diagnosis
multiple derivability
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multiple derivability
the strategy to infer a theory from evidence obtained by two or more independent methods that differ in the background
knowledge and technical principles on which they are based
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Mastitis streptococci and leukocytes (from Zschokke/Kitt 1908)
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mastitis diagnosis and underdetermination
somatic cell count of milk: false positive or false negative
(underdetermination)
bacteriological investigation: presence of bacteria without disease is possible
(underdetermination)
both tests positive >> mastitis
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mastitis diagnosis and undercalibration
electrical conductivity (EC) against SCC and bacteriological culturing (BC)
EC cheap and easy in robot milking
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meta analysis of sensitivity and specificity of EC for mastitis detection, using different gold standards
SSC sens 0.57 spec 0.94 BC sens 0.75 spec 0.95 SSC and BC sens 0.60 spec 0.91
Mirjam Nielen, thesis 1994, Utrecht University
mastitis diagnosis and undercalibration
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what to do in case of undercalibration? (i.e. that test results may be more or less probable than those of the reference test)
diagnosis
weighing evidence against context
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weighing evidence against context
epistemological context: background knowledge
social context: consequences of decision
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bovine mastitis weighing evidence against context
evidence: positive EC signal in milk of cow
social context: former mastitis problems on farm age of cow
stage of lactation
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underdiscrimination
frequency distributions, threshold values, sensitivity and specificity
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mastitis diagnosis and underdiscrimination
threshold values of somatic cell count (cells/ml milk)
going up and down by negotiation
Europe: 400,000 USA: 700,000
1950: 500,000 1970: 200,000 2009: 400,000
individual cow: 200,000 bulk tank milk: 400,000
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International Dairy Federation 1967
"It will be economically justified to fix the threshold value for cells in bulk farm milk in such a way, that not more than 10 % of the production of the milk
must be declared abnormal or mastitic and therefore be rejected for delivery to the market."
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diagnostic network
in which shifting of balances takes place
biomedical factors: SCC and immune system
technological factors: robot milking
commercial factors: milk quality
examples of factors that require re-framing of the network: