Study controls: Ashby responds.

Figure 1 is the relevant summary figure (Figure 8) from our article (Ashby et al. 2004). Our point in the article, as well as now, is that it is incumbent upon each investigator to accept, to study, and where possible, to understand the extent, nature, and origins of variability (within and between experiments) of the critical assay parameter. If you do not know why the assay parameter varies naturally with time, or between experiments, it becomes difficult to interpret small perturbations of the parameter induced in a chemical toxicity study. This was the problem we faced when we tried to explain our inability, over four extensive studies (Ashby et al. 2003), to confirm the effects that Sakaue et al. (2001) reported for bisphenol A (BPA). The control values for daily sperm production (DSP) in Sprague-Dawley rats over our four experiments (Figure 1) varied little, despite the use of three different rodent diets and a variety of physical test conditions (changes in bedding and caging). We also noted (Ashby et al. 2004) that Sakaue et al. reported similar control DSP values for Holtzman rats (Sakaue et al. 1999) and Sprague-Dawley rats (Sakaue et al. 2001; Figure 1). The most interesting aspect of the data in Figure 1 is the extent of variability in control DSP values reported by Sakaue et al. (2001) for their two experiments on BPA in Sprague-Dawley rats. It is important to understand the origins of these variations in control DSP values between similar experiments before interpreting small chemically induced perturbations in DSP values with confidence. Equally, by paying attention to the origins of control variability, we were able to show that two chemicals we had previously considered to be negative in the rodent uterotrophic assay were, in fact, weakly positive (Ashby et al. 2004). Stable control values for an assay lead to the generation of sound assay data.

[FIGURE 1 OMITTED]

The author is employed as a research scientist by Syngenta PLC.

REFERENCES

Ashby J, Tinwell H, Lefevre PA, Joiner R, Haseman J. 2003. The effect on sperm production in adult Sprague-Dawley rats exposed by gavage to bisphenol A between postnatal days 91-97. Toxicol Sci 74:129-138.

Ashby J, Tinwell H, Odum J, and Lefevre PA. 2004. Natural variability and the influence of concurrent control values on the detection and interpretation of low-dose or weak endocrine toxicities. Environ Health Perspect 112:847-853.

Ohsako S, Miyabara Y, Nishimura N, Kurosawa S, Sakaue M, Ishimura R, et al. 2001. Maternal exposure to a low dose of 2,3,7,8-tetrachlorodibennzo-p-dioxin (TCDD) suppressed the development of reproductive organs of male rats: dose dependent increase of mRNA levels of 5-alpha reductase type 2 in contrast to decrease of androgen receptor in pubertal ventral prostate. Toxicol Sci 60:132-143.

Sakaue M, Ohsako S, Ishimura R, Kurosawa S, Kurohmaru M, Hayashi Y, et al. 1999. Preliminary BPA experiment [Abstract]. In: 127th Annual Meeting of the Japanese Society of Veterinary Science, 2-4 April, Fujisawa, Japan. Abstract 47.

Sakaue M, Ohsako S, Ishimura R, Kurosawa S, Kurohmaru J, Hayashi Y, et al. 2001. Bisphenol A affects spermatogenesis in the adult rat even at a low dose. J Occup Health 43:185-190.

John Ashby

Syngenta Central Toxicology Laboratory

Alderley Park, Cheshire, United Kingdom

E-mail: [e-mail]