Cells that count. The standardizing of diagnostic tests for bovine mastitis Bert Nederbragt Descartes Centre for the History and Philosophy of the Sciences and the Humanities and Faculty of Veterinary Medicine

epidemiology clinical trial diagnosis therapy development pathogenesis

epistemology (and social framing) of a diagnostic test

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

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

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

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

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

epistemology of the diagnostic test bovine mastitis

epistemology of the diagnostic test decreased milk quality

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)

somatic cell count gold standard for subclinical mastitis

what to do in case of underdetermination? (i.e. that testresults may be false positives or negatives) diagnosis 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

Mastitis streptococci and leukocytes (from Zschokke/Kitt 1908)

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

mastitis diagnosis and undercalibration electrical conductivity (EC) against SCC and bacteriological culturing (BC) EC cheap and easy in robot milking

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

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

epistemological context: background knowledge social context: consequences of decision

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

underdiscrimination frequency distributions, threshold values, sensitivity and specificity

mastitis diagnosis and underdiscrimination threshold values of somatic cell count (cells/ml milk) going up and down by negotiation Europe: 400,000 USA: 700, : 500, : 200, : 400,000 individual cow: 200,000 bulk tank milk: 400,000

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."

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: