0.5 McFarland Standard: Values and Checks

Short answer

A 0.5 McFarland standard (also called a 0.5 McFarland turbidity standard) is a reference suspension used to set the cloudiness of a bacterial suspension to a known density. In the EUCAST disk diffusion method it corresponds approximately to 1-2 x 10⁸ CFU/mL for Escherichia coli. A spectrophotometer reading of the standard at 625 nm should fall in a narrow absorbance window, 0.08 to 0.13 for the barium sulfate version in the EUCAST manual.

What a 0.5 McFarland standard is

The McFarland scale is a set of numbered turbidity references. The original standards are made by adding barium chloride to sulfuric acid, which forms a fine barium sulfate precipitate; the more barium chloride, the higher the number. Hardy Diagnostics notes in its instructions for use that CLSI now also accepts latex particle suspension turbidity standards. The 0.5 point on the scale is the one most antimicrobial susceptibility methods call for when they ask for a standardised inoculum.

The standard does not count bacteria. It is a visual or photometric reference: a suspension that looks, or reads, as cloudy as the standard is treated as having roughly the density the method assumes. That is why the CFU/mL figure attached to it changes between documents (see the section on why the number varies below).

Barium sulfate versus latex standards

Both formats are used to match the same reference point. The differences below come from two primary documents: the EUCAST Disk Diffusion Manual v13.0 (January 2025, checked 2026-10-29) and the Hardy Diagnostics instructions for McFarland Latex Turbidity Standards (checked 2026-10-29).

Point	Barium sulfate (EUCAST Table 2)	Latex (Hardy Diagnostics IFU)
Composition	0.5 mL of 0.048 mol/L BaCl₂ (1.175% w/v BaCl₂.2H₂O) in 99.5 mL of 0.18 mol/L (1% v/v) H₂SO₄	Latex particle suspension
Absorbance check, 625 nm, 1 cm path	0.08 to 0.13	0.08 to 0.10 for the 0.5 standard
Stated density equivalent	Approximately 1-2 x 10⁸ CFU/mL for E. coli	Comparable to 1.5 x 10⁸ CFU/mL
Storage	Sealed tubes, in the dark, at room temperature	2-30 °C in the original container; do not freeze or overheat
Mixing	Mix thoroughly on a vortex mixer immediately before use (EUCAST Table 2, step 5)	Invert gently; do not vortex
Shelf life	Renew the standard or re-check its absorbance after 6 months (EUCAST Table 2, step 6)	Described as significantly longer than barium sulfate standards because of reduced light sensitivity

The latex product contains sodium azide as a preservative and is labelled for in vitro diagnostic use by trained personnel. The instructions say not to use it if there is deterioration, contamination, a colour change or if it is past expiry.

Using the standard to adjust an inoculum

In the EUCAST method, the isolate is suspended in saline to the density of a 0.5 McFarland standard. The manual gives an acceptable range of 0.4 to 0.6 McFarland and states the practical consequence: a denser inoculum gives reduced zones of inhibition and a lighter one the opposite. The suspension is adjusted by adding saline or more bacteria.

Photometer or visual comparison

EUCAST recommends a photometric device, calibrated against a 0.5 McFarland standard according to the manufacturer’s instructions. As an alternative the suspension can be compared by eye with the standard against a white background with black lines. Hardy Diagnostics describes the same reading: equal obliteration or distortion of the contrasting black lines indicates a turbidity match, and the suspension tube should have the same diameter as the standard tube. For background on the instrument side, see our guides to UV-Vis spectrophotometer basics and spectrophotometer calibration checks.

Timing

EUCAST says the suspension should optimally be used within 15 minutes and always within 60 minutes of preparation. Its footnote describes the 15-15-15 rule: use the suspension within 15 minutes, apply disks within 15 minutes of inoculating the plate, and incubate within 15 minutes of applying the disks.

Where the 0.5 level is not the target

For Streptococcus pneumoniae the manual asks for 0.5 McFarland when the growth comes from blood agar, but an inoculum equivalent to 1.0 McFarland (acceptable 0.9 to 1.1) when it comes from chocolate agar. The same standard is therefore not the right target for every organism and medium. The broth microdilution use of the 0.5 standard, followed by a further dilution, is covered in our guide to minimum inhibitory concentration by broth microdilution. For how susceptibility results are summarised across isolates, see building and reading a cumulative antibiogram, and for a related kinetic assay see time-kill assays.

The McFarland ladder

The Hardy Diagnostics latex standards give an approximate cell density and an absorbance specification at 625 nm for each number.

McFarland	Approx. cell density (x 10⁸/mL)	Absorbance at 625 nm
0.5	1.5	0.08-0.10
1	3	0.14-0.17
2	6	0.27-0.31
3	9	0.38-0.42
4	12	0.51-0.55
5	15	0.67-0.70
6	18	0.74-0.77
7	21	0.83-0.88
8	24	0.94-0.98

Why the CFU/mL figure varies

EUCAST gives approximately 1-2 x 10⁸ CFU/mL and ties it to E. coli; the latex instructions give 1.5 x 10⁸ CFU/mL. These are not in conflict. They describe different products and different specifications (an absorbance window of 0.08-0.13 versus 0.08-0.10), and the CFU/mL equivalent depends on the organism so the figure belongs to the method that states it. A laboratory should work to the number in the method it follows and use the standard as a turbidity reference, not as a colony count.

Bench checklist

Use a suspension tube of the same diameter as the standard tube (Hardy Diagnostics).
Read against a white card with black lines, in adequate light (both sources).
Calibrate the photometer against a 0.5 McFarland standard per its manufacturer’s instructions (EUCAST).
Accept 0.4 to 0.6 for the 0.5 target (EUCAST).
Use the suspension within 15 minutes where possible and within 60 minutes at the latest (EUCAST).
Invert, do not vortex, latex standards (Hardy Diagnostics).
Check the standard for deterioration, contamination, colour change and expiry before use (Hardy Diagnostics).

Where it matters for labs and supply decisions

Because the standard is a calibration reference, the useful questions are about format and compatibility rather than the number alone: which type of standard the method specifies, whether the reading device is validated for it, and how it is stored. Hardy Diagnostics states that use with instruments that rely on alternative light sources, such as scattered light, has not been established for its latex standards.

Common misunderstandings

For the wider field, see our plain-language guide to bacteriology.

“0.5 McFarland means exactly 1.5 x 10⁸ CFU/mL”

It is an approximate equivalence that depends on the organism, the product and the method. EUCAST words it as a range for E. coli.

“Broth colour does not affect the reading”

Dark yellow, orange or brown broth gives poor contrast for visual comparison, according to the Hardy Diagnostics instructions. The same document notes that very young Haemophilus influenzae colonies (under 24 hours) can give suspensions that are too heavy.

“A 0.5 standard works for every organism”

The S. pneumoniae chocolate agar case above shows it does not.

FAQ

What is a 0.5 McFarland turbidity standard?

A reference suspension, barium sulfate or latex, used to bring a bacterial suspension to the density that susceptibility methods such as EUCAST disk diffusion assume.

What absorbance should a 0.5 McFarland standard read?

At 625 nm with a 1 cm light path, 0.08 to 0.13 for the barium sulfate standard described by EUCAST, and 0.08 to 0.10 for the Hardy Diagnostics latex 0.5 standard.