Laboratory Procedures: free VTNE practice questions

The estimate is a single multiplication: viewed in the monolayer of the film under the 100x oil immersion lens, every platelet seen in a field represents roughly 15,000 per microliter, so 6 x 15,000 = 90,000 per microliter. Normal animals average at least seven platelets per field, and most domestic species run about 200,000 to 500,000 per microliter, so this dog is genuinely thrombocytopenic. The more useful part is what that number does and does not explain. Platelet numbers have to be extremely low before an animal bleeds spontaneously: the concentration generally has to fall below 20,000 per microliter for spontaneous hemorrhage, and below 50,000 per microliter to bleed from trauma. At 90,000 this dog's platelet count does not on its own account for the epistaxis, so the workup keeps going rather than stopping at the first abnormal number. Each wrong option is a specific error. 15,000 is the value one platelet represents, reported as though it were the count. 105,000 comes from multiplying by seven, the lower limit of normal per field, instead of the six actually counted. 900,000 is a decimal slip, multiplying by 150,000. One habit to build alongside the arithmetic: before trusting any low platelet count, scan the feathered edge of the film for platelet clumps. Clumping is the most common cause of a falsely low count, it is especially common in cats and in any sample that was difficult to draw or that sat for hours, and a clumped sample gives only a minimum platelet number, so a fresh sample is needed for an accurate one.

Laboratory errors sort into three groups, and knowing which group a control sample speaks to is what settles this. Preanalytical errors are everything that happens to the sample before it reaches the instrument. Analytical errors happen at the level of the test method itself, and they come from one of only two things: a substance or phenomenon present within the sample that interferes with the method, or the method's own performance failing. Postanalytical errors are transcription and other mistakes in generating and distributing the report. A quality-control program exists precisely to recognize and prevent failure of test method performance, so a control that ran alongside this patient and landed inside its limits is direct evidence that the method was working. That is what makes option A the tempting wrong answer - blaming the machine is the reflex - and it is also why it fails here.

Option B looks for a category that has become uncommon now that results move through automated laboratory information systems rather than being copied by hand.

Option C is the one to resist hardest: the reason for running through these error sources at all is that a result which is nonsensical or does not match the patient should not simply be believed. With the handling verified and the method vouched for by the control, what is left is something in the sample acting on the assay.