fix(labs): print range bars and lab values in the reader's number format

Range bars and the lab list, detail rows, chart tooltip and reference
ranges printed numbers with a plain `String()`, so a German reader saw
"1.8 mIU/L" and "61.3 kg" under a German interface. The same was true
of the mean, GMI, eA1C and the advanced indices on the glucose panel.

`RangeBar` now prints value, bounds and delta through the locale number
formatter and takes an optional `formatValue` for surfaces with their
own precision. Lab values keep their trim rule (whole numbers bare, at
most two decimals) in `formatLabValue`, which now hands the digits to a
caller-supplied locale formatter; `useLabNumber` wires it to the
reader's locale for every lab surface. Inputs are unchanged.

src/components/insights/glucose/__tests__/glucose-clinical-panel.test.tsx

@@ -162,6 +162,11 @@ describe("<GlucoseClinicalPanel> asserted state", () => {
     const de = render(<GlucoseClinicalPanel />, "de");
     expect(de).toContain("(3,9–10,0 mmol/L ist das Ziel)");
     expect(de).not.toMatch(/mg\/dl/i);
+    // Mean, GMI and eA1C read with the German decimal comma.
+    expect(de).not.toMatch(/>\d+\.\d+</);
+    expect(de).toMatch(
+      />\d+,\d<\/span><span class="text-muted-foreground text-xs">mmol\/L/,
+    );
 
     authMock.mockReturnValue({
       user: { glucoseUnit: "mg/dL" },

src/components/insights/glucose/glucose-advanced-disclosure.tsx

@@ -4,7 +4,7 @@ import { useId, useState } from "react";
 import { ChevronDown } from "lucide-react";
 
 import { InfoPopover } from "@/components/ui/info-popover";
-import { useTranslations } from "@/lib/i18n/context";
+import { useFormatters, useTranslations } from "@/lib/i18n/context";
 import { cn } from "@/lib/utils";
 import type { GlucoseClinicalMetrics } from "@/lib/analytics/glucose-metrics";
 import type { DataSummary } from "@/lib/analytics/trends";
@@ -49,11 +49,12 @@ export function GlucoseAdvancedDisclosure({
   fractionDigits = 1,
 }: GlucoseAdvancedDisclosureProps) {
   const { t } = useTranslations();
+  const number = useFormatters().number;
   const [open, setOpen] = useState(false);
   const regionId = useId();
 
   const fmt = (v: number | null) =>
-    v === null ? "—" : v.toFixed(fractionDigits);
+    v === null ? "—" : number(v, fractionDigits);
 
   const contexts = Object.entries(byContext ?? {}).filter(
     ([, s]) => s && s.count > 0,

src/components/insights/glucose/glucose-clinical-panel.tsx

@@ -82,7 +82,7 @@ export function GlucoseClinicalPanel() {
 
   const formatGlucoseValue = (mgdl: number): string => {
     const v = convertGlucose(mgdl, glucoseUnit);
-    return isMmol ? v.toFixed(1) : String(Math.round(v));
+    return fmt.number(v, isMmol ? 1 : 0);
   };
 
   // Learning state — calm, never asserting TIR / GMI off thin data.
@@ -189,7 +189,7 @@ export function GlucoseClinicalPanel() {
               icon={<Activity className="h-3.5 w-3.5" aria-hidden="true" />}
               label={t("insights.bloodGlucose.clinical.gmi.label")}
               help={t("insights.bloodGlucose.clinical.gmi.help")}
-              value={clinical.gmi.toFixed(1)}
+              value={fmt.number(clinical.gmi, 1)}
               unit="%"
             />
           ) : null}
@@ -198,7 +198,7 @@ export function GlucoseClinicalPanel() {
               icon={<TrendingUp className="h-3.5 w-3.5" aria-hidden="true" />}
               label={t("insights.bloodGlucose.clinical.eA1c.label")}
               help={t("insights.bloodGlucose.clinical.eA1c.help")}
-              value={clinical.estimatedA1c.toFixed(1)}
+              value={fmt.number(clinical.estimatedA1c, 1)}
               unit="%"
             />
           ) : null}

src/components/labs/__tests__/lab-reference-range-bar.test.tsx

@@ -7,9 +7,10 @@ import { LabReferenceRangeBar } from "../lab-reference-range-bar";
 
 function render(
   props: Partial<React.ComponentProps<typeof LabReferenceRangeBar>> = {},
+  locale: "en" | "de" = "en",
 ) {
   return renderToStaticMarkup(
-    <I18nProvider initialLocale="en">
+    <I18nProvider initialLocale={locale}>
       <LabReferenceRangeBar
         value={72}
         referenceLow={60}
@@ -45,4 +46,15 @@ describe("<LabReferenceRangeBar>", () => {
     expect(render({ referenceLow: null })).toContain('aria-label="≤100 bpm"');
     expect(render({ referenceHigh: null })).toContain('aria-label="≥60 bpm"');
   });
+
+  it("prints the reading and its range in the reader's number format", () => {
+    const html = render(
+      { value: 1.8333, referenceLow: 0.4, referenceHigh: 4.05, unit: "mIU/L" },
+      "de",
+    );
+    expect(html).toContain('aria-label="0,4–4,05 mIU/L"');
+    // Lab precision: at most two decimals, then the locale's separator.
+    expect(html).toContain("1,83 mIU/L");
+    expect(html).not.toMatch(/\d\.\d+ mIU\/L/);
+  });
 });

src/components/labs/biomarker-manager.tsx

@@ -13,12 +13,12 @@ import { ResponsiveSheet } from "@/components/ui/responsive-sheet";
 import { Skeleton } from "@/components/ui/skeleton";
 import { apiDelete, apiGet, apiPut } from "@/lib/api/api-fetch";
 import { formatReferenceRange } from "@/lib/labs/reference-range";
-import { formatLabValue } from "@/lib/labs/format-value";
 import { useTranslations } from "@/lib/i18n/context";
 import { queryKeys } from "@/lib/query-keys";
 
 import { BiomarkerForm } from "./biomarker-form";
 import type { BiomarkerDto, BiomarkerListResponse } from "./types";
+import { useLabNumber } from "./use-lab-format";
 
 /**
  * v1.18.1 — the Biomarker catalog manager. v1.22 — compact editable rows.
@@ -42,6 +42,7 @@ import type { BiomarkerDto, BiomarkerListResponse } from "./types";
  */
 export function BiomarkerManager() {
   const { t } = useTranslations();
+  const labNumber = useLabNumber();
   const queryClient = useQueryClient();
   const [formOpen, setFormOpen] = useState(false);
   const [editing, setEditing] = useState<BiomarkerDto | null>(null);
@@ -101,7 +102,7 @@ export function BiomarkerManager() {
     const range = formatReferenceRange(
       marker.lowerBound,
       marker.upperBound,
-      formatLabValue,
+      labNumber,
     );
     return (
       <li

src/components/labs/lab-biomarker-chart.tsx

@@ -16,12 +16,12 @@ import {
 import { Button } from "@/components/ui/button";
 import { CHART_HEIGHT_PX } from "@/lib/charts/constants";
 import { prefersReducedMotion } from "@/lib/charts/reduced-motion";
-import { formatLabValue } from "@/lib/labs/format-value";
 import { formatReferenceRange } from "@/lib/labs/reference-range";
 import { useTranslations, useFormatters } from "@/lib/i18n/context";
 
 import { RichChartTooltip, type RichTooltipRow } from "../charts/chart-tooltip";
 import type { LabResultDto } from "./types";
+import { useLabNumber } from "./use-lab-format";
 
 /**
  * v1.18.1 — per-biomarker trend chart, in the dashboard's visual language.
@@ -91,6 +91,7 @@ export function LabBiomarkerChart({
   upperBound: number | null;
 }) {
   const { t } = useTranslations();
+  const labNumber = useLabNumber();
   const fmt = useFormatters();
   const [range, setRange] = useState<RangeKey>("365");
   // Capture "now" once at mount so the recency filter stays pure across
@@ -224,7 +225,7 @@ export function LabBiomarkerChart({
                 tick={{ fontSize: 11, fill: "var(--muted-foreground)" }}
                 stroke="var(--border)"
                 width={44}
-                tickFormatter={(v: number) => formatLabValue(v)}
+                tickFormatter={(v: number) => labNumber(v)}
               />
               {/* Reference window — muted band + dashed bounds. No alarm
                   colour: the in/out verdict is shown by the neutral badge,
@@ -302,7 +303,7 @@ export function LabBiomarkerChart({
                   const rows: RichTooltipRow[] = [
                     {
                       name: t("labs.chart.valueLabel"),
-                      value: `${formatLabValue(point.value)} ${unit}`,
+                      value: `${labNumber(point.value)} ${unit}`,
                       color: primary,
                     },
                   ];
@@ -317,7 +318,7 @@ export function LabBiomarkerChart({
                         `${formatReferenceRange(
                           point.referenceLow,
                           point.referenceHigh,
-                          formatLabValue,
+                          labNumber,
                         )} ${unit}`.trim(),
                       color: secondary,
                     });

src/components/labs/lab-form.tsx

@@ -27,12 +27,12 @@ import {
   parseReferenceRange,
 } from "@/lib/labs/parse-reference-range";
 import { formatReferenceRange } from "@/lib/labs/reference-range";
-import { formatLabValue } from "@/lib/labs/format-value";
 import { EncounterSuggestionField } from "@/components/encounters/encounter-suggestion-field";
 import { useTranslations } from "@/lib/i18n/context";
 import { queryKeys } from "@/lib/query-keys";
 
 import { BiomarkerForm } from "./biomarker-form";
+import { useLabNumber } from "./use-lab-format";
 import type {
   BiomarkerDto,
   BiomarkerListResponse,
@@ -135,6 +135,7 @@ export function LabForm({
   footerSlot,
 }: LabFormProps) {
   const { t } = useTranslations();
+  const labNumber = useLabNumber();
   const recordName = useActiveRecordName();
   const queryClient = useQueryClient();
   const formId = useId();
@@ -325,11 +326,7 @@ export function LabForm({
   }
 
   const referenceText = selected
-    ? formatReferenceRange(
-        selected.lowerBound,
-        selected.upperBound,
-        formatLabValue,
-      )
+    ? formatReferenceRange(selected.lowerBound, selected.upperBound, labNumber)
     : "";
 
   // What the typed string resolved to, shown back so the user can see whether
@@ -342,7 +339,7 @@ export function LabForm({
       : `${formatReferenceRange(
           parsedSourceRange.low,
           parsedSourceRange.high,
-          formatLabValue,
+          labNumber,
         )} ${selected?.unit ?? ""}`.trim()
     : "";
 

src/components/labs/lab-history-list.tsx

@@ -38,6 +38,7 @@ import { ReferenceRangeBadge } from "./reference-range-badge";
 import { LabReferenceRangeBar } from "./lab-reference-range-bar";
 import { SourceRangeNote } from "./source-range-note";
 import type { LabResultDetailDto, LabResultDto } from "./types";
+import { useLabNumber } from "./use-lab-format";
 
 const NOTE_MAX_LENGTH = 2000;
 
@@ -77,6 +78,7 @@ function parseDecimal(raw: string): number | null {
  */
 export function LabHistoryList({ readings }: { readings: LabResultDto[] }) {
   const { t } = useTranslations();
+  const labNumber = useLabNumber();
   const { canManageDomain } = useRecordCapabilities();
   const canManageLabs = canManageDomain("labs");
   const queryClient = useQueryClient();
@@ -353,7 +355,7 @@ export function LabHistoryList({ readings }: { readings: LabResultDto[] }) {
           <li key={r.id} className="flex items-start gap-3 py-3">
             <div className="min-w-0 flex-1 space-y-2">
               <div className="font-semibold tabular-nums">
-                {formatLabReading(r)}
+                {formatLabReading(r, labNumber)}
               </div>
               <LabReferenceRangeBar
                 value={r.value}
@@ -421,7 +423,7 @@ export function LabHistoryList({ readings }: { readings: LabResultDto[] }) {
               <TableRow key={r.id}>
                 <TableCell className="font-semibold tabular-nums">
                   <div className="space-y-2">
-                    <span>{formatLabReading(r)}</span>
+                    <span>{formatLabReading(r, labNumber)}</span>
                     <LabReferenceRangeBar
                       value={r.value}
                       referenceLow={r.referenceLow}

src/components/labs/lab-list.tsx

@@ -15,13 +15,14 @@ import { Skeleton } from "@/components/ui/skeleton";
 import { apiGet } from "@/lib/api/api-fetch";
 import { formatDate } from "@/lib/format";
 import { formatReferenceRange } from "@/lib/labs/reference-range";
-import { formatLabReading, formatLabValue } from "@/lib/labs/format-value";
+import { formatLabReading } from "@/lib/labs/format-value";
 import { useTranslations } from "@/lib/i18n/context";
 import { cn } from "@/lib/utils";
 import { queryKeys } from "@/lib/query-keys";
 import { applyOrder, useModuleListPrefs } from "@/lib/module-list-prefs";
 
 import { LabTrendSparkline } from "./lab-trend-sparkline";
+import { useLabNumber } from "./use-lab-format";
 import { LabReferenceRangeBar } from "./lab-reference-range-bar";
 import { ReferenceRangeBadge } from "./reference-range-badge";
 import { SourceRangeNote } from "./source-range-note";
@@ -122,6 +123,7 @@ const MOBILE_CELL = {
 
 export function LabList({ onAddFirst }: { onAddFirst?: () => void } = {}) {
   const { t } = useTranslations();
+  const labNumber = useLabNumber();
   const { canWriteDomain } = useRecordCapabilities();
   const canAddLab = canWriteDomain("labs");
   const { prefs } = useModuleListPrefs("labs");
@@ -255,7 +257,7 @@ export function LabList({ onAddFirst }: { onAddFirst?: () => void } = {}) {
                   </div>
                   <div className="text-muted-foreground flex flex-wrap items-center gap-x-2 text-xs">
                     <span className="text-foreground font-semibold tabular-nums">
-                      {formatLabReading(group.latest)}
+                      {formatLabReading(group.latest, labNumber)}
                     </span>
                     <span>{formatDate(group.latest.takenAt)}</span>
                   </div>
@@ -373,7 +375,7 @@ export function LabList({ onAddFirst }: { onAddFirst?: () => void } = {}) {
                 <div className="flex items-end justify-between gap-3">
                   <div className="text-muted-foreground flex min-w-0 flex-1 flex-wrap items-center gap-x-2 text-sm">
                     <span className="text-foreground font-semibold tabular-nums">
-                      {formatLabReading(group.latest)}
+                      {formatLabReading(group.latest, labNumber)}
                     </span>
                     {group.latest.value !== null &&
                     (group.latest.referenceLow !== null ||
@@ -383,7 +385,7 @@ export function LabList({ onAddFirst }: { onAddFirst?: () => void } = {}) {
                         {formatReferenceRange(
                           group.latest.referenceLow,
                           group.latest.referenceHigh,
-                          formatLabValue,
+                          labNumber,
                         )}
                       </span>
                     ) : null}

src/components/labs/lab-reference-range-bar.tsx

@@ -2,6 +2,8 @@
 
 import { RangeBar } from "@/components/targets/range-bar";
 
+import { useLabNumber } from "./use-lab-format";
+
 /**
  * v1.37.34 — compact reference-range visualization for saved lab readings.
  *
@@ -20,6 +22,7 @@ export function LabReferenceRangeBar({
   referenceHigh: number | null;
   unit: string;
 }) {
+  const labNumber = useLabNumber();
   const hasLow = referenceLow !== null && Number.isFinite(referenceLow);
   const hasHigh = referenceHigh !== null && Number.isFinite(referenceHigh);
   if (
@@ -37,10 +40,10 @@ export function LabReferenceRangeBar({
   const min = referenceLow ?? referenceHigh! - scale;
   const max = referenceHigh ?? referenceLow! + scale;
   const rangeLabel = !hasLow
-    ? `≤${referenceHigh} ${unit}`
+    ? `≤${labNumber(referenceHigh!)} ${unit}`
     : !hasHigh
-      ? `≥${referenceLow} ${unit}`
-      : `${referenceLow}–${referenceHigh} ${unit}`;
+      ? `≥${labNumber(referenceLow!)} ${unit}`
+      : `${labNumber(referenceLow!)}–${labNumber(referenceHigh!)} ${unit}`;
 
   return (
     <div
@@ -55,6 +58,7 @@ export function LabReferenceRangeBar({
         unit={unit}
         compact
         tone="lab"
+        formatValue={labNumber}
         minLabel={referenceLow === null ? null : undefined}
         maxLabel={referenceHigh === null ? null : undefined}
       />

src/components/labs/source-range-note.tsx

@@ -1,10 +1,10 @@
 "use client";
 
-import { formatLabValue } from "@/lib/labs/format-value";
 import { formatReferenceRange } from "@/lib/labs/reference-range";
 import { useTranslations } from "@/lib/i18n/context";
 
 import type { LabResultDto } from "./types";
+import { useLabNumber } from "./use-lab-format";
 
 /**
  * The reference window a reading's own report printed, shown wherever that
@@ -40,6 +40,7 @@ export function SourceRangeNote({
   className?: string;
 }) {
   const { t } = useTranslations();
+  const labNumber = useLabNumber();
   if (reading.referenceOrigin !== "source") return null;
 
   const printed =
@@ -47,13 +48,13 @@ export function SourceRangeNote({
     `${formatReferenceRange(
       reading.referenceLow,
       reading.referenceHigh,
-      formatLabValue,
+      labNumber,
     )} ${reading.unit}`.trim();
 
   const catalog = formatReferenceRange(
     reading.catalogReferenceLow,
     reading.catalogReferenceHigh,
-    formatLabValue,
+    labNumber,
   );
 
   return (

src/components/labs/use-lab-format.ts

@@ -0,0 +1,18 @@
+"use client";
+
+import { useCallback } from "react";
+
+import { useFormatters } from "@/lib/i18n/context";
+import { formatLabValue, type LabNumberFormat } from "@/lib/labs/format-value";
+
+/**
+ * A lab value in the reader's number format, trimmed the way every lab
+ * surface trims it (whole numbers bare, at most two decimals).
+ */
+export function useLabNumber(): LabNumberFormat {
+  const fmt = useFormatters();
+  return useCallback(
+    (value: number) => formatLabValue(value, (n) => fmt.number(n)),
+    [fmt],
+  );
+}

src/components/targets/__tests__/range-bar.test.tsx

@@ -11,9 +11,12 @@ import { EDGE_PADDING_PERCENT, RangeBar } from "../range-bar";
  * extraction is provably side-effect-free.
  */
 
-function render(props: Parameters<typeof RangeBar>[0]) {
+function render(
+  props: Parameters<typeof RangeBar>[0],
+  locale: "en" | "de" = "en",
+) {
   return renderToStaticMarkup(
-    <I18nProvider initialLocale="en">
+    <I18nProvider initialLocale={locale}>
       <RangeBar {...props} />
     </I18nProvider>,
   );
@@ -93,4 +96,17 @@ describe("<RangeBar>", () => {
     expect(html).toContain('role="img"');
     expect(html).toContain('aria-label="Current: 72 bpm');
   });
+
+  it("prints value, bounds and delta in the reader's number format", () => {
+    // A German reader saw "61.3 kg" under a German interface.
+    const html = render(
+      { value: 72.4, min: 50.5, max: 70.25, unit: "kg" },
+      "de",
+    );
+    expect(html).toContain("50,5 kg");
+    expect(html).toContain("70,25 kg");
+    expect(html).toContain("72,4 kg");
+    expect(html).toContain("2,2 kg");
+    expect(html).not.toMatch(/\d\.\d+ kg/);
+  });
 });

src/components/targets/range-bar.tsx

@@ -6,7 +6,7 @@ import {
   TooltipProvider,
   TooltipTrigger,
 } from "@/components/ui/tooltip";
-import { useTranslations } from "@/lib/i18n/context";
+import { useFormatters, useTranslations } from "@/lib/i18n/context";
 
 /**
  * A horizontal range bar with green/yellow/red zones showing where
@@ -28,6 +28,12 @@ export interface RangeBarProps {
   tone?: "target" | "lab";
   minLabel?: string | null;
   maxLabel?: string | null;
+  /**
+   * How a value or bound is printed. Defaults to the reader's locale number
+   * format; a surface with its own precision rule (lab values trim to two
+   * decimals) passes it here so the bar and the row beside it agree.
+   */
+  formatValue?: (value: number) => string;
 }
 
 /**
@@ -49,8 +55,13 @@ export function RangeBar({
   tone = "target",
   minLabel,
   maxLabel,
+  formatValue,
 }: RangeBarProps) {
   const { t } = useTranslations();
+  const fmt = useFormatters();
+  // Every printed number goes through the locale formatter: a German reader
+  // sees "61,3 kg", not "61.3 kg".
+  const show = formatValue ?? ((v: number) => fmt.number(v));
   const isLab = tone === "lab";
 
   const span = max - min;
@@ -132,8 +143,8 @@ export function RangeBar({
   const deltaText =
     delta > 0
       ? value < min
-        ? t("targets.belowTarget", { delta: delta.toFixed(1), unit })
-        : t("targets.aboveTarget", { delta: delta.toFixed(1), unit })
+        ? t("targets.belowTarget", { delta: fmt.number(delta, 1), unit })
+        : t("targets.aboveTarget", { delta: fmt.number(delta, 1), unit })
       : t("targets.inTarget");
 
   // 2026-07-17 a11y audit (M1) — the marker's value + range live only in a
@@ -142,8 +153,8 @@ export function RangeBar({
   // 1.1.1 (no separate copy to drift) — `tabIndex` below makes the trigger
   // focusable, which is all Radix needs to also open the tooltip on focus.
   const markerAriaLabel = [
-    t("targets.currentValue", { value: String(value), unit }),
-    t("targets.targetRangeValue", { min: String(min), max: String(max), unit }),
+    t("targets.currentValue", { value: show(value), unit }),
+    t("targets.targetRangeValue", { min: show(min), max: show(max), unit }),
     deltaText,
   ].join(". ");
 
@@ -218,12 +229,12 @@ export function RangeBar({
             </TooltipTrigger>
             <TooltipContent>
               <p className="text-xs font-medium">
-                {t("targets.currentValue", { value: String(value), unit })}
+                {t("targets.currentValue", { value: show(value), unit })}
               </p>
               <p className="text-xs">
                 {t("targets.targetRangeValue", {
-                  min: String(min),
-                  max: String(max),
+                  min: show(min),
+                  max: show(max),
                   unit,
                 })}
               </p>
@@ -241,13 +252,13 @@ export function RangeBar({
           className="absolute -translate-x-1/2 whitespace-nowrap"
           style={{ left: `${minLabelPosition}%` }}
         >
-          {minLabel === null ? null : (minLabel ?? `${min} ${unit}`)}
+          {minLabel === null ? null : (minLabel ?? `${show(min)} ${unit}`)}
         </span>
         <span
           className="absolute -translate-x-1/2 whitespace-nowrap"
           style={{ left: `${maxLabelPosition}%` }}
         >
-          {maxLabel === null ? null : (maxLabel ?? `${max} ${unit}`)}
+          {maxLabel === null ? null : (maxLabel ?? `${show(max)} ${unit}`)}
         </span>
       </div>
     </div>

src/lib/labs/__tests__/format-value.test.ts

@@ -0,0 +1,25 @@
+import { describe, expect, it } from "vitest";
+
+import { makeFormatters } from "@/lib/format-locale";
+import { formatLabReading, formatLabValue } from "../format-value";
+
+const de = (n: number) => makeFormatters("de", "UTC", "AUTO", "AUTO").number(n);
+const en = (n: number) => makeFormatters("en", "UTC", "AUTO", "AUTO").number(n);
+
+describe("lab value formatting", () => {
+  it("trims to two decimals and prints the locale's separator", () => {
+    expect(formatLabValue(1.8, de)).toBe("1,8");
+    expect(formatLabValue(1.8, en)).toBe("1.8");
+    expect(formatLabValue(0.123456, de)).toBe("0,12");
+    expect(formatLabValue(5, de)).toBe("5");
+  });
+
+  it("formats a numeric reading with its unit and leaves a qualitative one verbatim", () => {
+    expect(
+      formatLabReading({ value: 61.3, valueText: null, unit: "kg" }, de),
+    ).toBe("61,3 kg");
+    expect(
+      formatLabReading({ value: null, valueText: "negativ", unit: "" }, de),
+    ).toBe("negativ");
+  });
+});

src/lib/labs/format-value.ts

@@ -5,11 +5,20 @@
  * lab reading the same way: whole numbers render bare, fractional values keep
  * up to two decimals (no trailing-zero noise). Centralised so the three
  * surfaces stay byte-identical.
+ *
+ * The digits themselves come from the caller's locale number formatter
+ * (`useFormatters().number`), so a German reader sees "1,8 mIU/L" where an
+ * English one sees "1.8 mIU/L". This module only decides the precision.
  */
-export function formatLabValue(value: number): string {
-  return Number.isInteger(value)
-    ? String(value)
-    : String(Number(value.toFixed(2)));
+export type LabNumberFormat = (value: number) => string;
+
+export function formatLabValue(
+  value: number,
+  formatNumber: LabNumberFormat,
+): string {
+  return formatNumber(
+    Number.isInteger(value) ? value : Number(value.toFixed(2)),
+  );
 }
 
 /**
@@ -18,13 +27,16 @@ export function formatLabValue(value: number): string {
  * formatted number plus its unit. One helper so the list card, the history
  * rows, and the detail header stay consistent.
  */
-export function formatLabReading(reading: {
-  value: number | null;
-  valueText: string | null;
-  unit: string;
-}): string {
+export function formatLabReading(
+  reading: {
+    value: number | null;
+    valueText: string | null;
+    unit: string;
+  },
+  formatNumber: LabNumberFormat,
+): string {
   if (reading.value === null) {
     return reading.valueText ?? "";
   }
-  return `${formatLabValue(reading.value)} ${reading.unit}`.trim();
+  return `${formatLabValue(reading.value, formatNumber)} ${reading.unit}`.trim();
 }