There are some significant differences between the two types of test method currently used for the detection of antibiotics in milk. Namely the rapid test (narrow spectrum) and the Inhibition test (broad spectrum).

Rapid Test – Delvotest BLF

Rapid tests use a specific receptor to bind an antigen (antibiotic) before it can cause a color change on the strip resulting in a positive or negative result. Being specific, means it has a narrow spectrum for accuracy and hence only detects one group of antibiotic at a time. The most commonly used test, targets the B-lactam group of antibiotic (see list on right) or more specifically, the B-lactam ring shown below. 

The B-lactam group is the most widely used antibiotics within the Dairy Industry, however it’s not the only one used. Due to antibiotic resistance and the fact that certain bovine diseases require different treatments, other antibiotic groups are also used.

Detecting various groups of antibiotics rapidly such as Tetracyclines, Macrolides, Sulfonamides, Aminoglycosides and others, a separate test kit or test strip is normally required. Although there are combination test kits available, they are normally limited to 3-4 groups and this adds considerably to the overall cost of each test performed.

The main advantage of these tests is the quick time it takes to get a result. Around 1 – 5 minutes meaning they are commonly used as a fast screening test for clearing milk tankers. This allows raw milk to be unloaded quickly into the silo at a processing site, hence their popularity. The majority of milk processors test milk tankers for B-lactam and confirm with the Delvotest SPNT and the benefit of the Delvotest BLF is the alignment on the detection limit within these tests.   

Microbial Inhibition Test

The microbial inhibition test is based on the principle that a very sensitive strain of bacterial spore (GeoBacillus stearothermophilus var calidolactis) is inhibited by the presence of antibiotics in milk when incubated at its optimum growth temperature of 64^oC. 

Unlike the rapid tests which can only detect one or 2 specific groups of antibiotics in a single test. The microbial inhibition test detects a broad range of antibiotics (such as B-Lactam, Tetracyclines, Macrolides, Sulfonamides, Aminoglycosides and others – see list on right) and thus provides better detection for the presence of antibiotics milk.

In a single test, it will detect all six major groups of antibiotics used in the Dairy Industry and is commonly used as a confirmatory test in Dairy Processing Plants. 

This is critical in plants that manufacture yoghurt and cheese as very low levels of antibiotics at or below the MRL in milk can inhibit the growth of starter cultures causing slow sets, runny yoghurt and lead to defects resulting in downgrades and product losses.

Microbial inhibition relies on the bacterial spore within the test medium to germinate and grow, generating acid which is indicated by a colour change from purple to yellow. This test takes at least 2 – 3 hours which is not a problem in farms or small to medium size dairy processors, but requires complex planning in larger ones.

The advantages are the broad range as explained above, but also the low cost per test, generally 50% - 80% lower than a rapid test depending upon the volume used. So what you lose in incubation time, you gain in detecting a broader range of antibiotics.