Milk pH is increased in lactating dairy cattle with
subclinical mastitis (SCM) and intramammary infection
(IMI). We hypothesized that milk pH testing provides
an accurate, low-cost, and practical on-farm method for
diagnosing SCM and IMI. The main objective was to
evaluate the clinical utility of measuring milk pH using
3 tests of increasing pH resolution: Multistix 10 SG
Reagent Strips for Urinalysis (Multistix strips, Bayer
HealthCare Inc., Elkhart, IN), pH Hydrion paper (Microessential Laboratory, Brooklyn, NY), and Piccolo
plus pH meter (Hanna Instruments, Woonsocket, RI),
for diagnosing SCM and IMI in dairy cattle. Quarter
foremilk samples were collected from 115 dairy cows at
dry off and 92 fresh cows within 4 to 7 d postcalving.
Quarter somatic cell count (SCC) was measured using a DeLaval cell counter (DeLaval, Tumba, Sweden),
with SCM defined as SCC >200,000 cells/mL and
IMI defined as SCC >100,000 cells/mL and the presence of microorganisms at ≥10 cfu/mL of milk. Milk
pH was measured at 37°C using the 3 test methods.
The Hydrion pH paper performed poorly in diagnosing SCM and IMI. Receiver operating curve analysis
provided optimal pH cutpoints for diagnosing SCM for
the pH meter (dry off, ≥6.67; freshening, ≥6.52) and
Multistix strips (dry off and freshening, ≥7.0). Test
performance of the pH meter and Multistix strips was
poor to fair based on area under the receiver operating
curve, sensitivity, specificity, positive likelihood ratio,
and kappa coefficient. The pH meter and Multistix
strips performed poorly in diagnosing IMI at dry off
and freshening. We concluded that milk pH does not
provide a clinically useful method for diagnosing SCM
or IMI in dairy cattle |
