Quality Assessment in the CSF Laboratory

2. The CSF/serum quotient is a method-independent value if derived after paired protein analysis of CSF and (appropriately diluted) serum samples.

If CSF and serum samples are analyzed in the same analytical run, compared to analysis with reference to two different calibration curves (and hence different accuracies), the coefficient of variation (CV) of CSF/serum quotients is also smaller (Andersson et al., 1994). However, a basic precondition must be fulfilled for these statements: the calibration curve must give concentration-independent accuracy. This is controlled by measuring a serially diluted serum sample with dilutions covering the complete analytical range of the corresponding calibration curve. With these preconditions, the quotient is independent of absolute accuracy of the CSF and serum concentration values, i. e., it is a method- and calibrator-independent value.

3. Q_Alb-related interpretations of immunoglobulin quotients are most specific for detection of an intrathecal immunoglobulin fraction in CSF if referred to a hyperbolic discrimination function in quotient diagrams.

In contrast to the linear IgG index or linear functions, which are subject to many false-positive interpretations (Reiber et al., 2001; Reiber et al., 2009; Stauch et al., 2010), the hyperbolic discrimination line is an empirically and theoretically founded function (Chap. 2).

4. The age-related evaluation of the Q_Alb to detect a barrier dysfunction must take into account the existence of different reference values for ventricular, cisternal, and lumbar CSF.

5. The integrated CSF report (Chap. 19) is part of a quality assessment because it adds reliability for the data analysis due to physiological plausibility of data combinations and clinical plausibility from disease-related patterns.

The physiological connection between different analyzed CSF parameters allows plausibility control of the value of any individual analyte if all the data from one patient are reported in a combined “integrated” data report (Chap. 19). This is an improvement for quality of results. With the disease-related data patterns of an integrated report we also gain reliability from complementary clinical information (differential diagnostic question of the clinician): any contradiction here indicates problems with the analytical results or with the differential diagnostic question.

Internal Quality Control for CSF proteins (Total Protein, Albumin, IgG, IgA, IgM)

CSF assays must take into account the low protein concentrations in normal CSF and subsequently the large variation of concentrations when intrathecal synthesis takes place, which is 10- to 100-fold larger than in the case of pathological changes in serum. This is the basic requirement made of industrial development: to allow sufficiently high sensitivity and a reasonable protocol, so that default dilution can be changed to avoid extreme positions on the calibration curve. These are the main problems in immunochemical nephelometry, in particular for IgM, that lead to concentration-dependent differences between the various analysis methods (Reiber et al., 2010). The individual laboratory has to control for this by internal quality control.

The use of an integrated CSF data report (Chap. 19; “Special Features for Quality of CSF Protein Analysis,” above) to report laboratory data to the clinician is a basic part of the quality assessment in the CSF laboratory: it improves quality control by adding plausibility control in the framework of disease-related data patterns. Discrepancies in the analytical data (Chap. 21) or discrepancies between the suggested diagnosis and the analytical results (Chap. 19) may prompt a repetition or extension of the analysis.

Precision. For interassay precision, CSF and serum control samples are measured with each analytical run and documented together with the CSF/serum quotients. Variation and trends between two calibrations can be read directly from the charts or by statistical calculation.

Accuracy. Control for accuracy of absolute values is ensured by two certified reference serum samples (normal range and pathological range) and two CSF samples in the upper and lower range of the calibration curve (see below). A calibration-dependent variability is recognized from mean values of the precision controls.

With each new assay or new batch of reagents, the laboratory must ensure method-independent accuracy of the paired CSF and serum analysis: a serum control sample is serially diluted down to CSF concentrations with dilutions covering the complete analytical range of the corresponding calibration curve in the CSF assay.

Control Material for CSF Analysis

Certified control samples for proteins in serum are used as available from commercial sources. However, to date (2010) no certified commercial protein control is available that is suited to normal CSF protein concentration (in particular for IgA and IgM), as the concentrations of analytes in commercial control samples are too high. This leads to serious control problems (Reiber et al., 2010) and makes the following particular proposals necessary.

Diluted control serum may be used (1:200 to 1:2000, depending on the analyte) and stored frozen in aliquots. In combination with a certified reference serum sample, an inhouse CSF pool may also be used with sample aliquots stored at 4°C, provided an antimicrobial agent has been added for stabilization (e. g., NaAzid).

A suitable control sample for normal CSF should yield (Andersson et al., 1994):

• IgM values between 0.5 and 1.5 mg/L.

• IgA values between 1.0 and 3.0 mg/L.

• IgG values between 10 and 30 mg/L.

• Albumin values between 100 and 300 mg/L.

The INSTAND Interlaboratory CSF Survey

Twenty years ago the interlaboratory CSF survey instituted by INSTAND in Germany established clinical interpretation of data patterns in addition to single result control. This EQA concept takes the postanalytical element—i. e., data interpretation—seriously as an aspect of general quality assessment. In this kind of training program for the interpretation of CSF data, participants also improve their alertness for methodological discrepancies. For a full account of the original survey, see Reiber (1995).