Titer (medicine)

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As titer , m, from French. titre = fineness (of gold), is used in biology and medicine to describe the volume of liquid that is just barely biological due to a biological substance ( agent ) dissolved in it .

The dimension of the titer is thus volume per amount of substance (V / n) or also volume per mass (V / m) or with dimension symbols L³ / N or L³ / M.

The titer is therefore the reciprocal of the chemical concentration , the measure of which is 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 conversely defined as the amount of the substance per unit volume, lies in the difficulty of quantifying some biological substances (e.g. antibodies , viruses , bacteria ) as such on their own, e.g. if one considers their mass or the amount of substance cannot or only with great difficulty determine. In contrast, their effects are often easier to determine. That is why the unit for the amount of substance is the amount that still has this specific effect, for example the just positive reaction in a certain test in the case of antibodies, just a lysis court in a bacterial lawn in the case of viruses , and just reproduction in a culture medium in the case of bacteria (e.g. Colititer ). The quantification is based on the absence (or falling below a certain value) of the effect of the substances if the sample volume used is too small.

Since it is usually difficult or impossible to measure volumes as small as is necessary, a dilution series is set up in practice and the same volume of the various dilutions is used for a standardized test. The most extensive dilution stage with effect - or, to put it another way, the smallest still effective volume of the original, undiluted sample liquid with which an effect can be achieved - is therefore in the foreground. This last dilution stage above the detection limit is also known as limit dilution . Accordingly, the liquid volume is in the numerator , the smallest unit of action as a reference value in the denominator of the fraction , which represents the measure for the content. In biology and medicine, the titer is defined as the volume of liquid per unit of action, i.e. inversely to the concentration (of the titer) in chemistry (amount per unit of volume). This also shows that the specification of the titer in biology only makes sense in connection with the specification of the test, i.e. together with the specification of the volume of the various sample dilutions used in the tests. Often only the degree of dilution is given as the titer. However, this information is only useful if it is clear what volume of diluted liquid the test is to be used with.

Procedure

medicine

The determination of the titer is a common method in medicine , for example to assess immunity after a vaccination or the increase in the concentration of antibodies during an acute infectious disease . Since in serology there is occasionally no purified or recombinant antigen available as a positive control and comparison value for a standard series , for example for a new pathogen , titers are then given as a dimensionless dilution level . As a rule - for example a blood serum - is diluted in two steps, i. H. dilutions of 1: 2, 1: 4, 1: 8, 1:16, 1:32, etc. are made. The dilutions are then z. B. on cell cultures , which are then infected with a virus . The highest dilution level at which infection of the cells is still completely prevented (i.e. sufficient antibodies are still present) is given as the titer. A reaction with a titer of 1: 1024 indicates a higher starting concentration than one with 1: 128, since a positive reaction of the test can still be determined despite the higher dilution, because the required concentration of the antibodies is still present in the dilution.

The indication of the titer has become less common today due to the preferred indication of mass concentrations and the development of other methods for antibody, antigen or virus determination. For some pathogens , a dilution series is still necessary, if z. B. a neutralization test or a complement fixation reaction must be carried out. However, when antibody concentrations are given in ng / ml or IU / ml in serology today , the term titer is used in the sense of chemistry .

microbiology

In microbiology , the titer method is often used to determine the concentration of microorganisms in aqueous liquids, for example water or beverages . For this purpose, a decimal dilution series of the sample is made, a certain volume of each dilution is added to a suitable culture medium and this is incubated. The culture approach inoculated with the highest dilution and still showing vegetation or a certain metabolic performance is determined . The titer can be determined from the dilution factor of the sample added to this batch and the volume added from the diluted sample to the culture medium. The dilution factor is the product of all dilution steps, for example, if the dilution is done in increments of ten (1:10), the dilution factor of the 4th dilution is 1 / 10,000 = 10 −4 .

Titer = volume of the diluted sample used × dilution factor

The microorganism concentration in the sample is the reciprocal of the titer, i.e. the higher the smaller the titer. For example, the titer 10 −6 mL corresponds to a microorganism concentration of 10 6 per mL.

If the sample contains a mixed microorganism community from different physiological types, this method often does not record all microorganisms, but only a selection of those types that multiply under the culture conditions used (including type of culture medium, temperature, access to oxygen). This is the case, for example, with samples from natural habitats such as water, soil and the like. This selectivity of the method is a disadvantage if one wants to record all microorganisms contained in the sample, but it can be used for a quantification of certain physiological types by using appropriate selective culture conditions. One example of this is the colititer .

A statistically secured variant of the titer method with the use of at least three parallel approaches is the “most probable number” method, the MPN method (English most probable number , MPN for short).

literature