View source for McFarland standards

{{Short description|Scale for turbidity measurement}}
{{More references|date=December 2009}}
[[File:McFarland standards.JPG|thumb|McFarland standards. No. 0.5, 1 and 2]]
In [[microbiology]], '''McFarland standards''' are used as a reference to adjust the [[turbidity]] of [[bacteria]]l [[Suspension (chemistry)|suspensions]] so that the number of bacteria will be within a given range to standardize microbial testing. An example of such testing is [[antibiotic]] [[Antibiotic sensitivity|susceptibility]] testing  by measurement of [[minimum inhibitory concentration]] which is routinely used in medical microbiology and research. If a suspension used is too heavy or too dilute, an erroneous result (either falsely resistant or falsely susceptible) for any given antimicrobial agent could occur.

Original McFarland standards were made by mixing specified amounts of [[barium chloride]] and [[sulfuric acid]] together. Mixing the two compounds forms a [[barium sulfate]] [[Precipitation (chemistry)|precipitate]], which causes turbidity in the solution. A 0.5 McFarland standard is prepared by mixing 0.05 mL of 1.175% barium chloride dihydrate (BaCl<sub>2</sub>•2H<sub>2</sub>O), with 9.95 mL of 1% sulfuric acid (H<sub>2</sub>SO<sub>4</sub>).<ref>{{Cite book|title=Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically; Approved Standard—Ninth Edition|last=Cockerill|display-authors=etal|first=Franklin R.|publisher=CLSI|year=2012|isbn=978-1-56238-784-6|pages=12}}</ref>  

Now there are McFarland standards prepared from suspensions of [[latex]] particles, which lengthens the shelf life and stability of the suspensions.
The standard can be compared visually to a suspension of bacteria in sterile [[saline (medicine)|saline]] or nutrient broth. If the bacterial suspension is too turbid, it can be diluted with more diluent. If the suspension is not turbid enough, more bacteria can be added.

McFarland [[nephelometer]] standards:{2}
{| class="wikitable"
|-
| McFarland Standard No.  || 0.5  || 1 || 2 ||  3 || 4
|-
| 1.0% [[barium chloride]] (ml) || 0.05 || 0.1 || 0.2 || 0.3 || 0.4
|-
| 1.0% [[sulfuric acid]] (ml)  || 9.95 || 9.9 || 9.8 || 9.7 || 9.6
|-
| Approx. cell density (1X10^8 CFU/mL) || 1.5 || 3.0 || 6.0 || 9.0 ||  12.0
|-
| % transmittance*    || 74.3 || 55.6 || 35.6 || 26.4 ||  21.5
|-
| Absorbance* || 0.08 to 0.1 || 0.257 || 0.451 || 0.582 || 0.669
|}
<nowiki>*</nowiki>at wavelength of 600&nbsp;nm

McFarland latex standards from Hardy Diagnostics (2014-12-10), measured at the UCSF DeRisi Lab:
{| class="wikitable"
|-
| McFarland Standard No.  || 0.5  || 1 || 2 || 4 || 6 || 8
|-
| Absorbance @600nm || 0.063 || 0.123 || 0.242 || 0.431 || 0.653 || 0.867
|}

== References ==
<references />
* THE NEPHELOMETER: AN INSTRUMENT FOR ESTIMATING THE NUMBER OF BACTERIA IN SUSPENSIONS USED FOR CALCULATING THE OPSONIC INDEX AND FOR VACCINES. JOSEPH McFARLAND, M.D. JAMA. 1907; XLIX(14):1176-1178.
Mcfarland Standards-http://www.dalynn.com/dyn/ck_assets/files/tech/TM53.pdf
{{Clinical microbiology techniques}}
{{DEFAULTSORT:Mcfarland Standards}}
[[Category:Microbiology equipment]]

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