Felis ISSN 2398-2950 Blood biochemistry: overview Contributor(s): Kathleen P Freeman Overview Sampling Tests Result Data Overview Measuring serum or plasma levels of nutrients, metabolites, enzymes, etc can provide general or specific data about organ function and disease processes. Commonly measured parameters include: Alanine aminotransferase (ALT) Blood biochemistry: alanine aminotransferase (SGPT, ALT). Albumin Blood biochemistry: albumin. Alkaline phosphatase (ALP) Blood biochemistry: alkaline phosphatase. Ammonia Blood biochemistry: ammonia. Aspartate aminotransferase (AST) Blood biochemistry: aspartate aminotransferase. Bicarbonate Blood biochemistry: bicarbonate. Bile acids Blood biochemistry: bile acid. Bilirubin - direct Blood biochemistry: direct bilirubin and total Blood biochemistry: total bilirubin. Calcium Blood biochemistry: total calcium. Chloride Blood biochemistry: chloride. Cholesterol Blood biochemistry: cholesterol. Creatinine Blood biochemistry: creatinine. Creatinine kinase (CPK) Blood biochemistry: creatine phosphokinase. Gamma globulins Blood biochemistry: gamma globulin. Gamma glutamyl transferase (GGT) Blood biochemistry: gamma glutamyl transferase. Globulin Blood biochemistry: total globulin. Glucose Blood biochemistry: glucose. Lactate dehydrogenase (LDH) Blood biochemistry: lactate dehydrogenase. Phosphate Blood biochemistry: phosphate. Potassium Blood biochemistry: potassium. Sodium Blood biochemistry: sodium. Total protein Blood biochemistry: total protein. Urea Blood biochemistry: urea. More specific tests Amylase Blood biochemistry: alpha amylase. C reactive protein. Folate Blood biochemistry: folate. Free fatty acids. Fructosamine Blood biochemistry: fructosamine. Glycerol. Glycosylated hemoglobin. Hormones: cortisol, thyroxine Thyroxine assay, parathyroid PTH assay. Iron Blood biochemistry: iron. Lead Lead toxicity. Lipase Blood biochemistry: lipase. Triglycerides. Trypsin-like immunoreactivity (TLI) Blood biochemistry: trypsin-like immunoreactivity. Vitamin B12 Blood biochemistry: vitamin B12. Sampling This article is available in full to registered subscribers Sign up now to purchase a 30 day trial, or Login Tests Methodologies A number of different methodologies may be available for any given test. Availability Many tests are routinely available. If in doubt, contact the laboratory to confirm availability of a specific test. ValiditySensitivity The ability of a test to produce a positive result in all animals that do have disease. A high sensitivity may be associated with a reduction in specificity as false positive reactions may occur. Specificity The ability of a test to produce a positive result only in animals that do have disease. A high specificity may be associated with a lower sensitivity as some false negative may occur. Predictive value How well the test performs in a given population of animals. Influenced by prevalence of the disease in the population being tested. Predictive value = how well the test performs in a given population of animals. Positive predictive value is probability of an abnormal test result indicating presence of disease. Negative predictive value is probability of a test result that is within normal limits corresponding to the absence of disease. Influenced by prevalence of the disease in the population being tested. Predictive value of positive test = p x sensitivity/p x sensitivity + (1-p) x (1-specificity). Predictive value of a negative test = (1-p) x specificity/(1-p) x specificity + P X (1-sensitivity). p = prevalence of disease. Example In a study of dogs with and without x-disease (diagnosed at necropsy) the following results are obtained: Positive with test : x-disease present 235 (TP), x-disease absent 16 (FP) - total 251. Negative with test: x-disease present 15 (FN), x-disease absent 200 (TN) - total 215. Total: x-disease present - 250, total x-disease absent -216. 1. Sensitivity = TPx100/TP+FN = 235x100/235+15 = 235/250x100 = 94%. 2. Specificity = TN/TN+FPx100 = 200/200+16x100 = 200/216x100 = 93%. It is estimated that x-disease occurs in about 20% of the population that will be tested. 3.Predictive value of a positive test = p(sens)/p(sens)+(1-p)(1-spec) = 0.20(0.94)/20(94)+(0.80)(0.07) = 0.1880/0.1880+0.0560 = 0.1880/0.2440 = 0.77. So you would expect disease in 77 out of every 100 with a positive test. If prevalence is only 2%: PVPT =0 .02(0.94)/0.02(0.94)+(0.80)(0.07) = 0.0188/0.0188+.0560 = 0.0188/0.0748 = 0.25. So you would expect to have x-disease in only 25 animals out of every 100 with a positive test. 4. Predictive value of a negative test (PVNT) = (1-p)(spec)/(1-p)(spec)+p(1-sens). If prevalence is 20%: (0.80)(0.93)/(0.80)(0.93)+0.20(0.06) = 0.7444/0.744+0.012 = 0.744/0.756 = 0.98. So would expect 98 out of every 100 animals with a negative test to be truly free of x-disease. If prevalence is 2%: (0.98)(0.93)/(0.98)(0.93)+(0.02)(0.06) = 0.9114/0.9114+0.0012 = 0.9114/0.9126 = 0.998. So would expect 99.8 out of every 100 dogs with a negative test to be truly free of x-disease. Technique (intrinsic) limitations In general, results of any given test are most significant when interpreted in conjunction with other laboratory and clinical findings. Result Data This article is available in full to registered subscribers Sign up now to purchase a 30 day trial, or Login Further ReadingPublicationsRefereed papers Recent references from VetMed Resource and PubMed. Levy J K, Crawford P C & Werner L L (2006) Effect of age on reference intervals of serum biochemical values in kittens. JAVMA 228(7), 1033-1037 PubMed. Other sources of information Kaneko J J, Harvey J W & Brass M L (eds) (1997) Clinical Biochemistry of Domestic Animals. 5th edn. Boston: Academic Press.