Short teaching lessons
Why can a positive veterinary test be wrong? Predictive value explained
Prepared 23 October 2026 · VetQ
Positive predictive value, or PPV, asks: among positive test results, how many are true positives? Even a test with strong sensitivity and specificity can produce many false positives when the condition is uncommon. Work through a population rather than memorising the phrase.
For veterinary exam study. These short examples do not prescribe treatment for an individual patient.
Use a fictional population
Suppose 1,000 animals include ten diseased and 990 nondiseased animals. A test with 90% sensitivity detects nine of the diseased animals. At 90% specificity, it falsely labels 99 nondiseased animals positive. There are 108 positive results in total.
Calculate the value
PPV = true positives ÷ all positives = 9 ÷ 108, approximately 8.3%. The test did not suddenly acquire a different sensitivity in this arithmetic example. The large nondiseased group generated many false positives relative to the small number of true positives.
Change prevalence while holding test characteristics fixed
If 100 of 1,000 animals are diseased with the same assumed sensitivity and specificity, there are 90 true positives and 90 false positives. PPV becomes 50%. The example illustrates a relationship under fixed assumptions, not a performance guarantee for a real test.
A second worked check: what is missing?
An invented question reports 70 true positives and thirty false positives. PPV is 70 ÷ 100 = 70%. Those numbers do not tell you specificity because the true-negative count is missing. They also do not tell you sensitivity because the false-negative count is missing. Write the denominator required for each statistic and mark which cells are available. If the question does not provide enough information, do not invent a population total. You can also reverse-check the PPV statement: among every hundred positive results in this fictional dataset, seventy are true positives. That verbal description should match the statistic you calculated.
Check your understanding
In a fictional dataset there are 40 true-positive and 10 false-positive results. What is PPV? Can you calculate sensitivity from those two numbers alone?
Read the answer and reasoning
PPV = 40 ÷ 50 = 80%. Sensitivity also needs the false-negative count, so it cannot be calculated from those numbers alone. Identify the denominator before choosing the statistic.
Keep reading
- Sensitivity explained with a veterinary exam example
- Specificity explained with a veterinary exam example
- Epidemiology and Preventive Medicine exam questions: topics, study tips and free sample questions
Sources and further reading
Source pages checked 23 October 2026. Examples and study routines on this page are original teaching material, not official examination items.