Which microscopic test does not have an agreed-upon procedure and standards?

Scanning Electron Microscopy (SEM) is recognized for its ability to provide highly detailed images of samples at the microscopic level. However, there is currently a lack of a standardized procedure and consensus when it comes to testing protocols and interpretation within the field of asbestos evaluation. This absence of uniformity in methodology can lead to variability in results, making SEM an unreliable choice for consistent asbestos analysis compared to other techniques that have established protocols.

In contrast, both Transmission Electron Microscopy (TEM) and Phase Contrast Microscopy are backed by generally accepted procedures and standards that guide their usage in asbestos assessments. TEM, for instance, is widely acknowledged as a reliable technique capable of identifying asbestos fibers down to the nanometer scale, thanks to its established guidelines. Similarly, Phase Contrast Microscopy is employed based on accepted practices, particularly for observing live biological samples. Light Microscopy also has set standards for asbestos identification, although it's less sensitive than TEM or SEM.

Thus, the distinct lack of agreed-upon procedures associated with SEM is what makes it the correct answer to the question, as it highlights an area within asbestos testing that is not yet fully standardized despite its potential capabilities.