Thanks for your question.
Most of the tests can be done manually with a pin prick, however for doing in automated machines we require venous blood. The pin prick blood tests which need to be done manually will be more inaccurate when compared to the automated blood tests. However some tests such as an ESR will require more quantity of blood which is not possible to get via a pin prick.
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Basically this has to do with the quantity of blood which if taken from a pin prick will not be sufficient for testing in an automated machine. Yes most of the tests done in a complete blood count may be done manually.
The inaccuracy is due to poor sample quality in the capillary pin prick blood.