The abundance of nanoparticles introduced to household products created the great expectations towards the application of nanotechnology in biology and medicine. That calls for cost-effective preliminary assessment of its cytotoxicity and biological activity. There are many attempts for creating proper guidance and standards for performing studies regarding nanoparticles. But still some important aspects crucial for in vitro testing of nanomaterials need more attention. Particulate nature is an obvious and widely unappreciated property of nanoparticles. In the context of in vitro studies, this property is critical, and it should be, but rarely is, considered when designing, performing, describing or interpreting the experiments involving the solid nanoparticles. First, we should be aware of relatively small and limited number of nanoparticles in the experimental setup. Even crude estimation of its number will be useful for proper interpretation of results. Second, we should not presume even distribution of particles in the solution, moreover we should expect that sedimentation and aggregation play an important role in interactions of nanoparticles with cells. In that case, expressing the dose in mass/volume units may lead as astray. Finally, the relation of size, weight, and number of nanoparticles makes comparisons of activity of nanoparticles of different sizes very complex. Estimations of number of nanoparticles in the dose should be an integral part of experiment design, its validation and interpretation.

纳米颗粒大量引入家用产品，引发了人们对纳米技术在生物学和医学领域应用的厚望。这就需要对其细胞毒性和生物活性进行具有成本效益的初步评估。人们进行了许多尝试来创建适当的指导和标准来进行有关纳米颗粒的研究。但对于纳米材料体外测试至关重要的一些重要方面仍然需要更多关注。颗粒性质是纳米颗粒的一个明显但广泛未被重视的特性。在体外研究中，这一特性至关重要，在设计、执行、描述或解释涉及固体纳米颗粒的实验时，应该考虑这一特性，但很少考虑。首先，我们应该意识到实验装置中纳米颗粒的数量相对较小且数量有限。即使粗略估计其数量也有助于正确解释结果。其次，我们不应该假设溶液中颗粒均匀分布，而且我们应该期望沉积和聚集在纳米颗粒与细胞的相互作用中发挥重要作用。在这种情况下，以质量/体积单位表示剂量可能会误入歧途。最后，纳米颗粒的尺寸、重量和数量之间的关系使得不同尺寸纳米颗粒的活性比较变得非常复杂。剂量中纳米粒子数量的估计应该是实验设计、验证和解释的一个组成部分。