Malaria in Returning Travelers

Diagnostics and Likelihood Ratios, Explained

Positive Findings (Patient Has This)

Finding (Sign/Symptom/Lab/Study)Number Needed to Diagnose
(Positive Likelihood Ratio)
Hyperbilirubinemia 7.3×
Splenomegaly 6.5×
Thrombocytopenia 5.6×
Fever 5.1×
Jaundice/icterus 4.5×
Pallor 2.8×
Hepatomegaly 2.4×
Vomiting 2.0×
Headache 1.8×
Chills/rigors 1.7×
Nausea 1.3×
Diarrhea 0.6×
Dyspnea 0.11×
Cough 0.04×

Negative Findings (Patient Doesn't Have This)

Finding (Sign/Symptom/Lab/Study)Number Needed to Diagnose
(Negative Likelihood Ratio)
Fever 0.12×
Thrombocytopenia 0.32×
Headache 0.40×
Chills/rigors 0.47×
Hyperbilirubinemia 0.65×
Splenomegaly 0.79×
Pallor 0.80×
Vomiting 0.86×
Nausea 0.88×
Jaundice/icterus 0.91×
Hepatomegaly 0.95×
Dyspnea 1.08×
Diarrhea 1.1×
Cough 1.3×


  • Outcome: Malaria
  • There were 14 studies conducted in malaria endemic areas, with adults, children, or adults and children populations. In 7 of 9 studies that species was clearly reported, P. falciparum predominated.
  • Most of the Pediatric studies were conducted at sub Saharan Africa. Other areas included India and Thailand.
  • Adult studies were mostly conducted at India and Tanzania. Children and Adult combined studies were conducted in Uganda, Zimbabwe, and the Philippines.
  • There were 7 studied investigating travelers returning from endemic areas. In 5 of 7 studies P. Falciparum predominated.


In returning travelers, the adequacy of malaria prophylaxis was not reported in a uniform fashion.


Rodrigo Kong, MD


March 15, 2012

What are Likelihood Ratios?

LR, pretest probability and posttest (or posterior) probability are daunting terms that describe simple concepts that we all intuitively understand.

Let's start with pretest probability: that's just a fancy term for my initial impression, before we perform whatever test it is that we're going to use.

For example, a patient with prior stents comes in sweating and clutching his chest in agony, I have a pretty high suspicion that he's having an MI – let's say, 60%. That is my pretest probability.

He immediately gets an ECG (known here as the "test") showing an obvious STEMI.

Now, I know there are some STEMI mimics, so I'm not quite 100%, but based on my experience I'm 99.5% sure that he's having an MI right now. This is my posttest probability - the new impression I have that the patient has the disease after we did our test.

And likelihood ration? That's just the name for the statistical tool that converted the pretest probability to the posttest probability - it's just a mathematical description of the strength of that test.

Using an online calculator, that means the LR+ that got me from 60% to 99.5% is 145, which is about as high an LR you can get (and the actual LR for an emergency physician who thinks an ECG shows an obvious STEMI).

(Thank you to Seth Trueger, MD for this explanation!)