Equis ISSN 2398-2977

# Blood: biochemistry - overview

Contributor(s): Ruth Morgan, Vetstream Ltd

## Sampling

to purchase a 30 day trial, or

## Tests

### Methodologies

• A number of different methodologies may be available for any given test.

It is important to be aware of which method is used so as to correctly interpret the results.

### 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.
• 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.