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Letter
Correct determination of critical flicker frequency is mandatory when comparisons to other tests are made
  1. Gerald Kircheis1,
  2. Norbert Hilger2,
  3. Dieter Häussinger1
  1. 1 Department of Gastroenterology, Hepatology and Infectious Diseases, Liver Center Düsseldorf, Heinrich Heine University Düsseldorf, Düsseldorf, Germany
  2. 2 Institute of Psychology, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany
  1. Correspondence to Dr Gerald Kircheis, Department of Gastroenterology, Hepatology and Infectious Diseases, Heinrich Heine University Düsseldorf, Moorenstrasse 5, Düsseldorf 40225, Germany; kircheis{at}med.uni-duesseldorf.de

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Dear Sirs,

There are many situations in daily life where we are exposed to flickering light. This is normally not appreciated because the flicker is so rapid that it is perceived as continuous light. The ability of the human visual system to identify flicker is dependent on the luminance and the chroma of the used stimulating light. The temporal resolution of the human photoreceptors in the central retina, where spatial resolution is optimal, is limited to critical flicker frequencies (CFF) below 50 Hz. Higher CFF values cannot be sensed and therefore are invisible to the naked eye due to the inertia of the human photoreceptors.1

Goldbecker et al 2 compared the four most favoured methods for diagnosing Hepatic Encephalopathy, including CFF. Their results suggest a low diagnostic value of CFF for detection and follow-up of manifest HE, which is in contrast to numerous other published studies (for review see3). Therefore apart from shortcomings related to study design (not randomised administration of the four different tests) and some inconsistencies regarding patient numbers and specificity calculations, we are concerned about the validity of the reported CFF results, which limits the conclusions drawn in this study.

Apparently 13 out of 86 patients of the control population have CFF values of 50 Hz or above (see figure 1D in the study by Goldbecker et al 2). Therefore about 15% of the reported CFF measurements in the control population are dubious. This also becomes evident when these values are compared either with the standard value tables of the SCHUHFRIED test system4 or the CFF distribution in 261 control persons in an own study (histogram in figure 1B). Figure 1A shows the histogram that was calculated from the data in figure 1D of Goldbecker et al.2 The error in measurement in the control group is obviously not random, as otherwise CFF values in the control population below the established borderline of 39 Hz had to be expected. Accordingly, a random error would result in a lower specificity. This is also not the case. Therefore one may conclude that the CFF measurements by Goldbecker et al reflect a systematic error. Most likely the CFFs found in the patients are similarly shifted to high values. Thus, the low sensitivity of CFF reported by Goldbecker et al does not reflect the performance of the test per se, but reflects an inadequate handling of the test by the authors. Too short dark adaptation periods may underlie these wrong CFF measurements. Under these conditions a shift of the red LED from a more dark red to a lighter red is misinterpreted by some probands as the beginning of flickering. However, CFF is the frequency at which light pulses are perceived as continuous flickering. In the article by Goldbecker et al no CFF data were shown for the cirrhotic population with manifest HE; we expect more erroneous measurements in analysing this population.

Figure 1

Distribution of CFF values in controls without liver disease. (A) Data derived from the paper of Goldbecker et al (n=86 aged 21 to 69 years; median 46.5 years) (B) Data from our own cohort (n=261 aged 13 to 78 years; median 49.0 years).

When test methods are compared with each other, it is important to guarantee the exactness of the performance of each method. This has apparently been missed in the case of CFF in the paper by Goldbecker et al 2 and resulted in erroneous conclusions.

References

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Footnotes

  • Contributors GK and NH: Conception, design, analysis, interpretation of data and drafting the article. DH: Interpretation of data, critical revision of the article for important intellectual content, and final approval of the version to be published.

  • Competing interests The authors (GK and DH) belong to a group of patent holders for a portable bedside device for critical-flicker-frequency analysis.

  • Ethics approval There was approval by the Ethics Committee of the Medical Faculty of Heinrich Heine University Düsseldorf for all single studies in Hepatic Encephalopathy including CFF measurements.

  • Provenance and peer review Not commissioned; internally peer reviewed.

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