diff --git a/src/components/labs/__tests__/lab-trend-sparkline.test.tsx b/src/components/labs/__tests__/lab-trend-sparkline.test.tsx
new file mode 100644
index 000000000..6b53ebb31
--- /dev/null
+++ b/src/components/labs/__tests__/lab-trend-sparkline.test.tsx
@@ -0,0 +1,82 @@
+/**
+ * Which colour each reading earns, for every shape a reference window can
+ * arrive in.
+ *
+ * The case worth spelling out is the inverted pair. A floor at or above its
+ * ceiling is not a one-sided window, it is a window that cannot be true, and
+ * the tempting reading — "well, we still have a minimum" — paints a high value
+ * green. That is the wrong direction for a transcription error to fail in, so
+ * it earns no verdict at all, matching what `lab-reference-range-bar.tsx` does
+ * with the same input.
+ */
+import { describe, expect, it } from "vitest";
+import { renderToStaticMarkup } from "react-dom/server";
+
+import { LabTrendSparkline } from "../lab-trend-sparkline";
+
+const READINGS = [10, 50, 150];
+const MUTED = [
+  "fill-muted-foreground",
+  "fill-muted-foreground",
+  "fill-muted-foreground",
+];
+
+/** The fill class of each plotted reading, oldest → newest. */
+function fills(
+  referenceLow: number | null,
+  referenceHigh: number | null,
+): string[] {
+  const markup = renderToStaticMarkup(
+    <LabTrendSparkline
+      values={READINGS}
+      referenceLow={referenceLow}
+      referenceHigh={referenceHigh}
+    />,
+  );
+  // Each reading is a small rect, not a circle — this PR squares them off to
+  // match the reference-range bar's marker.
+  return Array.from(markup.matchAll(/<rect[^>]*class="([^"]*)"/gu)).map(
+    (m) => m[1],
+  );
+}
+
+describe("a reading's colour against its reference window", () => {
+  it("judges against a real band", () => {
+    expect(fills(30, 100)).toEqual([
+      "fill-info",
+      "fill-success",
+      "fill-warning",
+    ]);
+  });
+
+  it("judges a floor-only window", () => {
+    expect(fills(30, null)).toEqual([
+      "fill-info",
+      "fill-success",
+      "fill-success",
+    ]);
+  });
+
+  it("judges a ceiling-only window", () => {
+    expect(fills(null, 100)).toEqual([
+      "fill-success",
+      "fill-success",
+      "fill-warning",
+    ]);
+  });
+
+  it("gives no verdict without a window", () => {
+    expect(fills(null, null)).toEqual(MUTED);
+  });
+
+  it("gives no verdict on an inverted pair rather than reading it as a floor", () => {
+    // 100/30 — the same numbers as the valid band, transposed. Read as
+    // "minimum 100" this would paint 150 green and 50 flagged.
+    expect(fills(100, 30)).toEqual(MUTED);
+  });
+
+  it("gives no verdict when floor and ceiling are equal", () => {
+    // A zero-width band cannot separate "inside" from "outside" either.
+    expect(fills(50, 50)).toEqual(MUTED);
+  });
+});
diff --git a/src/components/labs/lab-trend-sparkline.tsx b/src/components/labs/lab-trend-sparkline.tsx
index 8d25e767a..e2e297a08 100644
--- a/src/components/labs/lab-trend-sparkline.tsx
+++ b/src/components/labs/lab-trend-sparkline.tsx
@@ -63,10 +63,16 @@ export function LabTrendSparkline({
       if (value > referenceHigh) return "fill-warning";
       return "fill-success";
     }
-    if (referenceLow !== null) {
+    // Each one-sided branch requires the OTHER bound to be absent. Without
+    // that, a present-but-impossible pair (floor at or above ceiling) reads as
+    // "minimum only" and paints a high value green — the wrong direction for a
+    // transcription error to fail in. Falling through to the neutral dot
+    // matches `lab-reference-range-bar.tsx`, which renders nothing at all for
+    // the same input: a range that cannot be true earns no verdict.
+    if (referenceLow !== null && referenceHigh === null) {
       return value < referenceLow ? "fill-info" : "fill-success";
     }
-    if (referenceHigh !== null) {
+    if (referenceHigh !== null && referenceLow === null) {
       return value > referenceHigh ? "fill-warning" : "fill-success";
     }
     return "fill-muted-foreground";