McFarland standards - Misplaced Pages

(Redirected from McFarland standard) Scale for turbidity measurement

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed.
Find sources: "McFarland standards" – news · newspapers · books · scholar · JSTOR (December 2009) (Learn how and when to remove this message)

In microbiology, McFarland standards are used as a reference to adjust the turbidity of bacterial suspensions so that the number of bacteria will be within a given range to standardize microbial testing. An example of such testing is antibiotic 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 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 (BaCl₂•2H₂O), with 9.95 mL of 1% sulfuric acid (H₂SO₄).

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 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}

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

*at wavelength of 600 nm

McFarland latex standards from Hardy Diagnostics (2014-12-10), measured at the UCSF DeRisi Lab:

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

Cockerill, Franklin R.; et al. (2012). Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically; Approved Standard—Ninth Edition. CLSI. p. 12. ISBN 978-1-56238-784-6.

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

Techniques in clinical microbiology

Isolation
and culture

Isolation techniques	Asepsis Streak plate Selective media
Cultures by body site	Blood culture Genital culture Sputum culture Throat culture Urine culture Wound culture
Cultures by organism	Bacterial culture Fungal culture Viral culture

Identification
and testing

Manual testing: basic techniques	Colonial morphology Hemolysis Staining Gram stain Acid-fast stain Giemsa stain India ink stain Ziehl–Neelsen stain Wet prep Rapid tests Oxidase Catalase Indole PYR
Manual testing: biochemical and immunologic tests	ALA test Amino acid decarboxylase test Bile solubility test CAMP test Citrate test Coagulase test DNAse test IMViC KOH test Methyl red test Nitrite test ONPG test Oxidative/fermentation glucose test Phenylalanine deaminase test Reverse CAMP test Salt tolerance test Sulfide indole motility test Triple sugar iron test Urease test rapid Voges–Proskauer test X and V factor test Bacitracin susceptibility test Optochin susceptibility test Novobiocin susceptibility test Lancefield grouping RPR test
Automated and point-of-care testing	Analytical profile index MALDI-TOF Polymerase chain reaction VITEK Rapid strep test Monospot test
Antibiotic susceptibility testing	Beta-lactamase test Disk diffusion test Etest McFarland standards Minimum inhibitory concentration

Equipment

This standards- or measurement-related article is a stub. You can help Misplaced Pages by expanding it.

Categories: