Canis ISSN: 2398-2942

Blood biochemistry: overview

Contributor(s): Kathleen P Freeman

Overview

Measuring serum or plasma levels of nutrients, metabolites, enzymes, etc can provide general or specific data about organ function and disease processes. Print-off the owner factsheet on 'Samples - how they help your vet' to give to your client.

Sampling

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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.

Validity

Sensitivity

  • 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 speicificity 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

  • Predictive value = how well the test performs in a given population of animals.
  • Influenced by the prevalence of the disease in the population being tested.
  • 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.
  • 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+0.0560 = 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

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Further Reading

Publications

Refereed papers

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.


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