Antibody Titres and Two Types of Bias
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In this chapter, two types of possible bias for antibody titres are explored. The first type of bias is due to how antibody titres are defined. An alternative definition is presented, the mid-value definition. With this definition, the bias is properly corrected. The second type of bias occurs when antibody titres above (or below) a certain level are not determined. If this bias is ignored, the geometric mean titre will be underestimated. It is demonstrated how the method of maximum likelihood estimation for censored observation can be applied to eliminate this bias.
4.1 Standard Antibody Titres Versus Mid-Value Titres
There are antibody assays that try to correct for this bias. An example is the interpolated serum bactericidal assay (SBA) to demonstrate humoral immune responses induced by meningococcal vaccines. Meningococcal disease is caused by the bacterium Neisseria meningitidis, also known as meningococcus bacteria. Attack rates of the disease are highest among infants aged younger than two years and adolescents between 11 and 19 years of age. The disease can cause substantial mortality. Five serogroups, A, B, C, Y and W135, are responsible for virtually all cases of the disease. The standard SBA titre is defined as the reciprocal of the highest dilution of serum immediately preceding the 50% survival/kill value for colony-forming units (50% cut-off). The interpolated SBA titre is calculated using a formula that determines the percentage kill in dilutions on either side of the 50% cut-off. The titre is the reciprocal of the dilution of serum at the point where the antibody curve intersects the 50% cut-off line.
The log-transformed antibody titres are normally distributed.
The dilutions are predefined.
The distance between two consecutive log-transformed dilution factors is small compared to the range of observed log-transformed titre values.
For post-vaccination titres, this is usually the case. (For pre-vaccination titres, often condition 1 or 3 is not met.)
4.2 Censored Antibody Titres and Maximum Likelihood Estimation
If the number of dilutions in an antibody assay is limited, then it may happen that at the highest tested dilution the assay read-out did not occur. In that case, often the titre is set to the reciprocal of the highest dilution. The result of this practice is bias. The geometric mean titre will be underestimated, and the assumption of normality for the distribution of the log-transformed titres may not hold.
4.2.1 ML Estimation for Censored Normal Data
A censored observation is an observation for which a lower or an upper limit is known but not the exact value. An example of a censored observation is a value below the detection limit of a laboratory test. The upper limit of the test result is known, the detection limit, but not the test result itself. If for an observation only an upper limit is known, the observation is called left-censored. Observation for which only a lower limit is known is called right-censored. Censored observations are often assigned the value of the limit. If this is not taken into account in the statistical analysis, estimates will be biased. A powerful statistical method to eliminate this bias is maximum likelihood (ML) estimation for censored data. As an introduction to ML estimation for censored antibody titres, in this section ML estimation for censored normal data is discussed.
Maximum likelihood estimation for censored observations was introduced for the analysis of survival data, where it is used to handle censored survival times .
4.2.2 ML Estimation for Censored Antibody Titres
To obtain ML estimates for censored, log-transformed antibody titres, SAS procedure PROC LIFEREG can be used. This is a procedure to fit probability distributions to data sets with censored observations. A wide variety of distributions can be fitted, including the normal distribution.
Before the log-likelihood function for censored observations can be applied to log-transformed antibody titres, a modification is needed. In the previous section, it was assumed that the data were censored normal observations. Log-transformed antibody titres, however, are not continuous observations; they are so-called interval-censored observations. An interval-censored observation is an observation for which the lower and the upper value is known but not the exact value. If this is not taken into account, i.e. if the values are treated as if continuous, the ML estimates PROC LIFEREG returns will be invalid.
(continued) The starting dilution was 1:4, and thus antibody titres less or equal to 4 are to be considered as left-censored. Below a SAS code to fit a normal distribution to the censored log-transformed antibody titres in Fig. 4.1 is given.
Two parameters are estimated, an intercept, which is the ML estimate of \(\mu \), and a scale parameter, which is the ML estimate of \(\sigma \). For both parameters, two-sided 95% confidence limits are given. Note that the \(\mu \) estimated is the mean of the distribution underlying the log-transformed mid-value titres (Sect. 4.1), and not the mean of the distribution underlying the log-transformed standard titres. This is due to the values assigned to the SAS variables Lower and Upper, which are consistent with the mid-value definition. To estimate the \(\mu \) consistent with the definition of standard titres, in the above SAS code Lower should be set to Logtitre−0.5 and Upper to Logtitre \(+\) 0.5. Then SAS Output 4.1B is obtained.
SAS Code 4.1 Fitting a normal distribution to the censored antibody titres of Fig. 4.1
SAS Output 4.1A
The ML estimate of \(\mu \) is now consistent with the standard titre definition. (This value could have of course also been obtained by subtracting 0.5 from the ML estimates in SAS Output 4.1A: \(6.227-0.5 = 5.727\).) Note that the correction does not have an effect on the ML estimate of \(\sigma \).
SAS Output 4.1B
The ML estimates 5.73 and 2.39 are in good agreement with the estimates based on the uncensored data, 5.80 and 2.49. This demonstrates the powerful tool ML estimation for censored observations is.
Above as log transformation, the standard transformation \(\log t = \log _2 [t/(D/2)]\) was used, with D the starting dilution factor. A general SAS code to fit a normal distribution to the censored \(\log _e\) transformed serial twofold antibody titres is presented below.
SAS Code 4.2 Fitting a normal distribution to censored serial twofold antibody titres
The approach discussed above can be readily extended to the case of two vaccine groups. Let Group be the SAS variable for the groups, taking the value 1 for the experimental group and 0 for the control group. Then the MODEL statement in SAS Code 4.1 must be changed to