Titer

As titer is called in biology and medicine that volume of fluid due to a biological substance dissolved in it ( agent ) just yet developed a biological activity.

The unity of the titre is therefore volume per mole (V / n) or volume per mass (V / m).

The titer is therefore the reciprocal of the chemical concentration has been made, the amount or mass per volume (m / V).

Areas of application, principle of determination

The reason for the deviation from the concept of concentration, which is inversely defined as the amount of the substance per unit volume, is the difficulty of some biological substances to quantify (such as antibodies, viruses, bacteria) itself as such, as if their mass or amount of substance can not or very difficult to determine. Their effects, however, are often more easily identifiable. Therefore, one takes as the unit of amount of substance that amount which just has this specific effect, such as antibodies, just positive response in a particular test, in viruses just a Lysishof in a lawn of bacteria in bacteria just propagation in a culture medium (eg Colititer ). The quantification is based on the absence of (or below a certain value ), the effect of the substance when the volume of sample used is too small.

Since it is usually difficult or even impossible, to gauge small volume, as it is required in the practical implementation of a dilution series of each of the same volume of various dilutions is created and for a standardized test used. The most extensive dilution with effect - or the other way around say the least yet effective volume of the original, undiluted sample liquid, to produce an effect can be achieved - thus stands in the foreground. This last dilution step above the limit of detection is also called limiting dilution. Accordingly, the volume of liquid in the counter, the smallest unit of activity is as a reference value in the denominator of the fraction, which is the measure of the content. The titer is defined in biology and medicine as the volume of liquid per unit of action, ie inversely to the concentration ( the titer ) in chemistry ( amount per unit volume ). It also shows that the indication of the titre in biology makes sense only in the context of the specification of the tests, ie together with the details of which volume of the different sample dilutions used in the test. Often expressed as titer only the degree of dilution. This information is only useful if it is clear with what volume of the diluted liquid, the test is applied.

Method

Medicine

The determination of the titer in medicine a conventional method, for example, to evaluate an immunity after a vaccination or the increase in the concentration of antibodies during acute infectious disease. As in serology occasionally not purified or recombinant antigen is as positive control and reference value for a standard range available, for example when a new pathogen titers are then reported as dimensionless dilution step. Is normally - for example, a blood serum - diluted in two stages, i.e. dilutions of 1:2, 1:4, 1:8, 1:16, 1:32, etc. produced. The dilutions are then added, for example, to cell cultures are then infected with a virus. The highest dilution in which even infection of the cells is completely prevented (that is still sufficient antibody is present), is expressed as titer. A reaction at a titer of 1:1024 there is a higher initial concentration of as one in 1:128, since despite higher dilution, a positive response of the test is to determine more or less as antibodies still sufficient for positive reaction.

An indication of the titre is becoming less common, due to the preferred indication of the mass concentrations and the development of other methods of antibody, antigen or virus determination today. For some pathogens, a dilution series is still necessary, such as when a neutralization test or a complement fixation test must be performed. If now be given in the serology antibody concentrations in ng / ml or IU / ml, the expression titers but is used in the sense of chemical.

Microbiology

In Microbiology Titerverfahren the often used to determining concentrations of microorganisms in aqueous fluids, for example water, drinks. To a decimal dilution series of the sample is prepared, if a specific volume of each dilution in a suitable culture medium and incubated for this. It determines the medium inoculated with the highest dilution and even vegetation and of a given metabolic power displayed culture approach. Of the dilution factor in this approach, the given sample and given from the diluted sample in the culture medium volume, the titer can be determined. The dilution factor is the product of all dilution steps, diluted, for example in increments of ten (1:10), so is the dilution factor of 4 dilution 1/10000 = 10-4.

Titer = volume of the diluted sample used x dilution factor

The microorganism concentration in the sample is the reciprocal value of the titre, thus the higher, the lower the titre. For example corresponds to the titer of 10-6 mL of a microorganism concentration of 106 per mL.

If the sample contains a mixed microorganisms Company in various physiological types, they are often not detected by this method, all microorganisms, but only a selection of those types that proliferate under the applied culture conditions (including type of culture medium, temperature, oxygen access ). This is for example the case with samples from natural habitats such as water, soil and the like. This selectivity of the method is a disadvantage if one wants to capture all the microorganisms present in the sample, but it can be used for quantification of certain physiological types by appropriate selective culture conditions are employed. An example of this is the Colititer.

A statistically significant variation of Titerverfahrens with the use of at least 3 replicates the method of the "most probable number ", the MPN method is (english most probable number, abbreviated MPN ).