We read with great interest the article from Haubelt et al (2005) on the variables influencing performance of the platelet function analyser (PFA)-100® test system (Dade Behring, Germany) and were surprised by some of the conclusions and recommendations.
The authors stated correctly that, for calculation of normal ranges and cut-off values, confirmation of normal platelet function in control subjects is essential. Moreover, diagnostic cut-offs derived partly from individuals with pathological results will affect the clinical performance of the test by increasing false-negative cases. Thus, the collagen/adrenaline (CEPI) cartridge screen can easily be affected by numerous medications (Koscielny et al, 2004).
The reported upper cut-off for the CEPI cartridges in Haubelt et al (2005) (95th percentile: 223 s) was much higher than the cut-off quoted in product labelling (95th percentile: 165 s) and by other reports in the literature. In contrast, the reported lower cut-off for the CEPI as well as both cut-offs for the collagen/ADP cartridge agreed with product labelling guidance.
To justify the deviation in the CEPI cut-off, the authors claimed to have applied the best methods to ensure normal platelet function in their selected subjects. If this was the case, any samples demonstrating mild platelet dysfunction [and higher closure time (CT)] would have been excluded and calculated cut-offs expected to be lower than reported in other papers and product labelling, not higher.
A more likely explanation for this discrepancy is that samples with mild platelet dysfunction, because of unreported drug intake, thyroid hormones supplementation (Mamiya et al, 1989) or dietary components, have affected the database. In fact, when looking at the aggregometry data, reported ranges varied from low response (32%) to very high response (114%), suggesting that some samples had aggregation results at or below levels of 30%.
In case the reported CEPI cut-off is, in fact, too high, many conclusions of the paper need to be interpreted with caution.
The conclusion that the PFA-100® performance would necessitate duplicate testing is doubtful. When looking at the effect of duplicate testing on the clinical accuracy (Mammen et al, 1998), only a very moderate benefit was achieved compared with single testing. Considering that the PFA-100® results are interpreted in a semi-quantitative way – a numerical result interpreted as either normal or abnormal platelet function – duplicate testing is not required. Instead, optimised preanalytical settings, appropriate system maintenance and repeated testing only when results do not agree with clinical history, are better recommendations.
In more general terms, some clear international directives on standardisation of platelet function testing, including the required preanalytics, are desperately needed.
Haubelt et al (2005) also nicely described and confirmed the effect of several well known variables on platelet function as measured with the PFA-100® test system, such as the significantly longer CTs of ABO type-O individuals, a difference that is most probably attributable to the lower von Willebrand factor (VWF) levels. As the authors pointed out, the PFA-100® system is highly dependent on VWF levels. In fact, the lower VWF-related platelet function in ABO type-O might indicate a lower risk for cardiovascular events (von Beckerath et al, 2004) and be detectable only with VWF-dependent platelet function tests.
The authors also confirmed that the platelet function was slightly higher in the morning with shorter PFA-100® CTs, as well as by bleeding time (BT) and platelet aggregation (PA). In fact, many intrinsic factors, including coagulation and fibrinolytic factors, show diurnal variations.
However, the recommendation of Haubelt et al (2005), that this would imply that PFAs can only be used during a limited time window during the day, is disputable. Shorter CT in the morning might well indicate a higher than day-average platelet function and should be measured and interpreted as such. As the clinical implications of the higher platelet function in the early morning are still unclear, the findings do warrant further studies for a better understanding of this phenomenon.
The reported difference in CT found between males and females is interesting, especially when considering the demographic data of these groups. Females show significantly higher body mass index values (Davi et al, 2003), while the prevalence of ABO type-O was much lower compared with the male group. Both of these gender differences should have been discussed as possible reasons for the CT difference. Based on our own experience and results from a larger controlled study (Böck et al, 1999), we do not believe that males and females express different platelet function as measured with the PFA-100®.
Finally, Haubelt et al (2005) found no relation between the PFA-100® and BT or PA. One must keep in mind that the BT, PA and PFA-100® are different systems with different rheological parameters and milieus. The clinical relevance of each test system and their correlation therefore depends very much on the question and background clinical context.
In our experience, as well as that of other groups, in addition to the detection of clear and clinically relevant platelet functional defects, we established that the PFA-100® is more sensitive than the BT to detect aspirin intake and VWF deficiencies, which are the most frequent acquired and congenital cause of primary haemostasis deficiency respectively.