**Question:** In adults, the blood culture ratio of blood to reagent is -

A. 1:10
B. 1:5
C. 1:100
D. 1:1000

**Core Concept:** Blood culture is a microbiological test used for the isolation and identification of microorganisms from blood samples. In order to obtain an accurate result, a specific ratio of blood to reagent is crucial.

**Why the Correct Answer is Right:** The correct answer is **B. 1:5**. This ratio ensures that there is sufficient blood to detect microorganisms, while also minimizing the risk of contamination. By adding a smaller volume of blood to a larger volume of reagent, we increase the sensitivity (ability to detect microorganisms) without compromising specificity (ability to distinguish contaminants from pathogens).

**Why Each Wrong Option is Incorrect:**

A. 1:10 (10 ml blood to 100 ml reagent): This ratio is too low, increasing the risk of contamination and reducing sensitivity.

C. 1:100 (1 ml blood to 10 ml reagent): This ratio is too high, potentially leading to an insufficient number of microorganisms being detected, lowering the overall sensitivity.

D. 1:1000 (0.1 ml blood to 10 ml reagent): Similar to option C, this ratio is too high, compromising the sensitivity of the blood culture.

**Why Correct Option is Right:**

A well-balanced ratio, such as option B, ensures that a sufficient volume of blood is present for microorganism detection while minimizing the risk of contamination. This allows for optimal sensitivity and specificity in identifying potential infections.

**Clinical Pearl:** In practical terms, it is recommended to use a 10 ml blood sample and 50 ml reagent for blood culture, following the ratio of 1:5. This ensures accurate results in diagnosing infections and guiding appropriate antimicrobial therapy.