Reagent lot comparison

Run this study#

What you are checking: whether a new reagent or calibrator lot changes any patient result by more than your laboratory allows.

Suggested starting plan: 3 concentration groups × 20 independent patient specimens, each measured once on both lots: 60 specimens, 120 results. Choose groups that cover your reporting range and decision points. Base the count on the method's variability. For example, with a paired-difference SD of about 2 units, 20 specimens per group give a 95% interval of about ±0.94 units around the mean difference (NIST mean-interval formula). The calculation minimum is 2 specimens with a completed plan.

1. Decide first. Choose New study, pick Reagent lot comparison, name both lots, enter the Required matched pairs (how many patient specimens you plan) and choose Create study. Then decide the allowable difference for each specimen (in the study unit, as a percentage of the previous-lot result, or both; or choose Use a CLIA / CAP limit) and the concentration groups. Enter them on Set up (see Study setup).
2. Measure both lots. Run the same patient specimens on both lots within their stability window, with calibration and QC in control. Enter repeats under the same specimen ID. They are averaged.
3. Enter results: specimen ID, lot, result and level. Lot and level names must match the study exactly. IDs starting with QC are control material and are summarized separately. On Data, choose Import a file and choose your file (start from Blank template (CSV) or lots.csv). Answer any question the dialog asks, then choose Import into this study.
4. Calculate and review. Choose Calculate results. Results opens. Check each specimen's outcome under New lot … vs previous lot …, then Planned vs completed specimens. Check zero previous-lot results under combined limits. Fix any unmatched specimen or uncovered group. On Report, choose Download PDF.

What you get: the lot passes when every specimen is within its allowable difference, at least 2 specimens are included and the plan is complete. Mean differences and confidence intervals are shown for context.

Purpose#

One study compares one pair of lots. Differences are new lot minus previous lot. A negative difference means the new lot reads lower.

When to use it#

Use it when a new lot arrives for the same analyzer. For two methods or analyzers, use Method comparison or Instrument comparison. For imprecision alone, use Repeatability or Multi-day precision.

Study setup#

1. Choose New study. Enter Analyte and Unit, select Reagent lot comparison under Comparison and bias, and fill in Previous lot name, New lot name and Required matched pairs (how many patient specimens you plan to measure on both lots). Choose Create study, or Create and import data if you already have results.
2. On Set up, check Previous lot and New lot under Study details. Under Lot plan, check Required matched pairs. Lot plan analyte fills in from the study. Under Planned concentration groups, choose Add concentration group for each level. Enter its Level as it appears in your data, its Required pairs and, optionally, Range low and Range high in the study unit. The range is checked against previous-lot results.
3. Under Acceptance limits, choose Enter your own limit and enter the allowance for each specimen. See Statistics and exact acceptance for how it is applied.

To reuse the plan in another study with the same analyte and unit, choose Save as a lot plan under Lot plans, name it and choose Save lot plan. In the other study, pick it under Saved lot plan and choose Load lot plan. A saved lot plan carries no acceptance limits. Set them in each study, or copy them with Reuse limits from a saved set or another study.

Entering data#

On Data, choose Import a file, or Add row and type or paste into the grid. Blank template (CSV) downloads the columns this study needs.

Fix these before calculating:

• a marked row, such as a lot name that matches neither lot
• no allowable difference under Acceptance limits
• a plan with no required pairs or no concentration group
• a plan for another analyte or unit
• a specimen in more than one level
• no patient specimens, only QC rows

Statistics and exact acceptance#

For each specimen, X is its previous-lot result and Y its new-lot result, after averaging replicates. With an absolute allowance A in the study unit and a percentage allowance P:

• Difference = Y − X.
• A percentage allowance is a percentage of X. It needs a positive X and a scale with a meaningful zero.
• Allowance = A, or X × P / 100, or with both limits, as chosen under With both limits: max(A, X × P / 100) for Pass when within either (the larger allowance), min(A, X × P / 100) for Pass only when within both.
• The specimen meets the limit when |Y − X| ≤ allowance.
• Error index = difference ÷ allowance. Values from −1 to +1 meet a symmetric limit.

Every specimen must meet its limit, whatever the mean difference.

Differences are compared with the limit at full precision. A difference exactly on the limit passes.

The percentage summaries in the results file (mean, SD and mean interval of the percentage differences) use a different denominator: 100 × (new − previous) / mean of the two lot results. For 20 → 22 and 100 → 98 they average 3.75%, while the same differences as a percentage of the previous lot average 4%. Only the per-specimen limit above decides the lot.

Sources and the numerical checks behind these calculations: Methods and sources.

Worked example#

lots.csv has six synthetic glucose specimens in two groups and one QC material, each measured on LOT-A and LOT-B, 14 rows in all. The example project's study Glucose · reagent lot comparison uses it.

Every specimen is Met, and both groups have 3 of 3 pairs inside their planned range. The study status is Criteria met. The mean difference is +0.37 mg/dL, with a 95% CI of −0.47 to 1.20.

With a ±1 mg/dL limit, S04 (+1.30 mg/dL) fails and the study status is Criteria not met, although the mean difference is well inside ±1. If S06's new-lot result is excluded instead, S06 can't be evaluated, the plan has 5 of 6 pairs, and the study is Undecided.

Reading the results#

New lot … vs previous lot … lists every included specimen, unmatched ones too, under Specimen decisions. If something else stops the lot passing, such as an incomplete plan, that reason comes first.

Planned vs completed specimens shows whether each planned group is covered, with its counts and ranges.

Supporting statistics and QC holds the mean differences by level, imprecision and QC summaries.

The report adds an error-index plot of each specimen's difference ÷ allowance against its previous-lot result. Points inside the shaded band from −1 to +1 are within the limit.

Common corrections#

• Unmatched specimen: check the specimen ID and lot spelling. Enter the missing result or exclude the row with a reason.
• Uncovered group: check the level name, then add specimens until the group has its planned count within its range.
• Failed specimen: review preparation, stability, instrument and lot records. Correct a mistyped result in its row. Excluding a result asks for a reason.