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The objectives of this experiment were to determine interrelationships among mastitis indicators and evaluate the subclinical mastitis detection ability of infrared thermography (IRT) in comparison with the California Mastitis Test (CMT). Somatic cell count (SCC), CMT, and udder skin surface temperature (USST) data were compiled from 62 Brown Swiss dairy cows (days in milk=117+/-51, milk yield=14.7+/-5.2 kg; mean +/- SD). The CORR, REG, and NLIN procedures of Statistical Analysis System (SAS Institute Inc., Cary, NC) were employed to attain interrelationships among mastitis indicators. The diagnostic merit of IRT as an indirect measure of subclinical mastitis was compared with CMT using the receiver operating characteristics curves. The udder skin surface temperature was positively correlated with the CMT score (r=0.86) and SCC (r=0.73). There was an exponential increase in SCC (SCC, x10(3) cells/mL=22.35 x e(1.31 x CMT score); R(2)=0.98) and a linear increase in USST (USST, degrees C=33.45+1.08 x CMT score; R(2)=0.75) as the CMT score increased. As SCC increased, USST increased logarithmically [USST, degrees C=28.72+0.49 x ln(SCC, x10(3) cells/mL); R(2)=0.72]. The USST for healthy quarters (SCC <or=400,000 cells/mL; n=94) was different from that for subclinical mastitic quarters (SCC >400,000 cells/mL; n=135) (mean +/- SE; 33.45+/-0.09 vs. 35.80+/-0.08 degrees C). The sensitivity, specificity, positive likelihood ratio, negative likelihood ratio, positive predictive value, and negative predictive value were 95.6, 93.6, 14.97, 0.05, 95.0, and 93.6, respectively, for IRT and 88.9, 98.9, 83.56, 0.11, 99.2, and 86.1, respectively, for CMT. The area under the receiver operating characteristics curve for IRT and CMT was not different. In conclusion, as a noninvasive and quick tool, IRT can be employed for screening subclinical mastitis via measuring USST, with a high predictive diagnostic ability similar to CMT when microbiological culturing is unavailable. However, the reliability of IRT among cows with different characteristics and those living under various environmental conditions remains to be determined.