The microhematocrit tube is a specialized, narrow-bore capillary tube used to determine the Packed Cell Volume (PCV), also known as the hematocrit. This measurement quantifies the proportion of blood volume occupied by red blood cells after centrifugation. Precision in this diagnostic step provides practitioners with important information about conditions like anemia or polycythemia.
Minimum Tubes Required Per Sample
While a single microhematocrit tube theoretically holds enough blood for one determination, standard laboratory procedure mandates the use of duplicates for every patient sample. This requirement ensures the reliability of the reported result by providing a direct check on the measurement process.
The two resulting hematocrit values must fall within a narrow, acceptable range, typically defined as 1 to 2 percentage points of each other. If the duplicates agree, the average of the two readings is reported as the final, confirmed hematocrit value. This duplication is a simple yet effective quality control measure against errors in sampling or reading the centrifuge scale.
Accounting for Reproducibility and Technical Errors
The practical reality of laboratory work dictates stocking more than the minimum two tubes per sample to account for common technical challenges and reproducibility failures. One frequent issue involves the physical handling of the delicate glass tubes, which can easily break during the sealing process using clay or wax, or during the high-speed centrifugation step. Furthermore, if the initial blood collection is marginal, the subsequent filling of the duplicate tubes might result in one or both having an insufficient column of packed cells for accurate reading.
A significant procedural failure occurs when the initial duplicate readings are too disparate, exceeding the acceptable 1-2% variance. This disagreement invalidates both initial readings, requiring a retest to determine the true value. In such cases, a third or even a fourth tube must be readily available to run another set of duplicates or a “tie-breaker” measurement.
For these reasons—breakage, insufficient sample volume, and non-reproducible results—a prudent laboratory typically allocates three, and often four, microhematocrit tubes for every patient sample being processed. This reserve stock reduces the need to recall the patient or redraw the sample, streamlining the workflow and maintaining laboratory efficiency.
Calculating the Total Tube Supply Needed
Shifting focus from a single sample to a full laboratory or teaching environment requires a systematic approach to calculating the total inventory of tubes needed for a batch run. The total supply calculation must factor in both the patient samples and the necessary quality control (QC) procedures.
The number of tubes allocated per patient should align with the risk assessment from the previous section, often using the conservative figure of three or four tubes. For instance, if a laboratory processes 20 patient samples and allocates three tubes per sample to account for errors, that requires 60 tubes for patient analysis alone.
Beyond patient samples, every batch run must include quality control (QC) samples—blood samples with known, verified hematocrit values—to ensure the equipment and procedure are performing correctly. Typically, at least two QC samples (one low and one high value) are run in duplicates alongside the patient samples.
If two QC samples are run, requiring two tubes each, this adds four tubes to the total requirement. For example, a batch of 20 patients, allocating three tubes per patient, requires 64 tubes total (60 patient tubes + 4 QC tubes). This systematic method ensures the laboratory is prepared for both routine analysis and technical failures.