Positive Predictive Value Versus Likelihood Ratio As A Trigger For Primary Care Cancer Evaluation

What symptoms and signs should lead a primary care physician to evaluate a patient for possible cancer. I’ve been reviewing this question for a number of cancers (Resources 1, 2, 3, 4, and 5 below).

Resource (1) states that they used a threshold positive predictive value for symptom/symptoms of 3% as the trigger for recommending a primary care evaluation for cancer.

But Resource (3) uses the likelihood ratio of symptoms as the trigger for a primary care physician to evaluate for cancer.

The resources below address this issue.

Resource (6) discusses the difference between positive and negative predictive value and the likelihood ratio:

Two important measures of test performance
are positive predictive value (PPV), the proportion
of patients with positive test who actually have the
disease, and negative predictive value (NPV), the
proportion of patients with negative test who are
actually free of the disease. These measures are
usually represented as percentages.

Although clinicians are well familiar with
predictive values, these measures are not
invariant characteristics of the tests and
significantly depend on the prevalence of the
disease in the population tested. In order to solve
this problem, the other measure can be used as
Likelihood Ratio (LR) which is independent of
prevalence[3,4]

LR is one of the most clinically useful measures.
LR shows how much more likely someone is to get
a positive test if he/she has the disease, compared
with a person without disease. Positive LR is
usually a number greater than one and the
negative LR ratio usually is smaller than one.

Although LR is very useful and some authors
have proposed simple methods to use this
criterion, there are several limitations to using it
in clinical practice. To use this measure a
nomogram should be employed or pretest
probabilities should be converted into Odds, then
multiplied by LR, then converted back into post
test probability (Post-test odds = pre-test odds*
LR)[5,6].

Resources:

(1) The 2015 Suspected cancer: recognition and referral, NICE guideline [NG12] Published date: June 2015. [Link is to the full guideline pdf (378 pages).

(2) Some Excerpts and Resources From The 2015 Guideline “Suspected Cancer: Recognition and Referral” Posted on February 10, 2017 by Tom Wade MD

(3) Diagnosis of Lung Cancer – Help From The American Family Physician With Additional Resources Posted on February 16, 2017 by Tom Wade MD

(4) Diagnosis Of Ovarian Cancer In Primary Care – Help From The American Family Physician With Additional Resources Posted on February 19, 2017 by Tom Wade MD

(5) Pancreatic Cancer Diagnosis In Primary Care Posted on February 24, 2017 by Tom Wade MD

(5) Interpretation of Diagnostic Tests: Likelihood Ratio vs. Predictive Value [PubMed Abstract] [Full Text HTML] [Full Text PDF]. Iran J Pediatr. 2013 Dec; 23(6): 717.

(6)Evidence Based Emergency Medicine Part 1: Simple Definition and Calculation of Accuracy, Sensitivity and Specificity [PubMed Abstract] [Full Text HTML] [Full Text PDF]. Emerg (Tehran). 2015 Spring;3(2):48-9.

(7) Evidence Based Emergency Medicine Part 2: Positive and negative predictive values of diagnostic tests [PubMed Abstract] [Full Text HTML] [Full Text PDF]. Emerg (Tehran). 2015 Summer;3(3):87-8.

(8) Evidence Based Emergency Medicine Part 3: Positive and Negative Likelihood Ratios of Diagnostic Tests [PubMed Abstract] [Full Text HTML] [Full Text PDF]. Emerg (Tehran). 2015 Fall;3(4):170-1.

(9) Evidence Based Emergency Medicine; Part 4: Pre-test and Post-test Probabilities and Fagan’s nomogram [PubMed Abstract] [Full Text HTML] [Full Text PDF]. Emerg (Tehran). 2016 Winter;4(1):48-51.

(11) Episode 62 Diagnostic Decision Making in Emergency Medicine [Link is to the show notes and the podcast] from Emergency Medicine Cases.

 

Posted in Guidelines, Laboratory Tests, Medical Decision Making, Medical Imaging, Medical Tests | Comments Off on Positive Predictive Value Versus Likelihood Ratio As A Trigger For Primary Care Cancer Evaluation