Understanding the 2 × 2 table
Before calculating the positive predictive value (PPV), the results must be arranged into a standard diagnostic accuracy table. The point-of-care test is compared against the laboratory hemoglobin test, which serves as the reference standard.
| Point-of-care result | Anemia present (lab positive) | Anemia absent (lab negative) | Total |
|---|
| Test positive | 85 (true positive) | 48 (false positive) | 133 |
| Test negative | 15 (false negative) | 352 (true negative) | 367 |
| Total | 100 | 400 | 500 |
What PPV actually means
Positive predictive value answers the clinical question:
If a pregnant woman has a positive point-of-care hemoglobin test, what is the probability that she truly has anemia? It is calculated only among women who tested positive.
PPV = true positives ÷ all positive tests × 100 =
85 ÷
133 × 100 =
63.9%
The denominator includes both true positives and false positives because every positive result—whether correct or incorrect—is what the clinician must act on. A false positive means a woman without anemia is told she may be anemic, which can lead to unnecessary iron supplementation or further testing.
Why PPV is not a fixed property of the test
PPV depends on the prevalence of anemia in the tested population. In this sample, anemia prevalence is
100 ÷
500 =
20%. At this prevalence, the PPV is
63.9%. If the same point-of-care device were used in a population with higher anemia prevalence, the PPV would rise; in a lower-prevalence population, it would fall. This is a key distinction from sensitivity and specificity, which are intrinsic test characteristics.
Key point! Sensitivity and specificity describe the test itself. PPV and NPV describe how the test performs in a specific population. Do not confuse PPV with sensitivity.
Clinical relevance in rural and antenatal settings
Point-of-care hemoglobin testing is particularly valuable where laboratory access is limited. In low- and middle-income settings, anemia is a substantial contributor to poor pregnancy outcomes, and on-site testing can expand screening coverage
[1]. Devices such as the HemoCue and Masimo Rad-67 have been evaluated in antenatal care settings precisely because timely hemoglobin results allow same-day clinical decisions
[2][4]. However, the presence of false positives—
48 women in this sample—means that a positive point-of-care result should be interpreted as a screening trigger rather than a definitive diagnosis.
Watch out! A PPV of
63.9% means roughly
36 out of every
100 positive results are false positives. In practice, confirmatory laboratory testing or repeat hemoglobin measurement may be warranted before initiating long-term treatment, especially when iron therapy carries side effects or when resources are constrained
[3][4].
Calculation steps for the exam
1. Identify true positives:
85 (point-of-care positive and lab-confirmed anemia).
2. Identify all positive tests:
85 +
48 =
133.
3. Divide true positives by all positives:
85 ÷
133 =
0.639.
4. Multiply by 100 and round to one decimal place:
63.9%.
The other options represent common errors:
26.6% is the false positive rate among all positive tests (
48 ÷
180 would be incorrect; actually
48 ÷
133 =
36.1%, which is option 2), and
88.0% is the negative predictive value (
352 ÷
367). Option 2 (
36.1%) is the complement of PPV—the proportion of positive tests that are false positives—so selecting it reflects a misunderstanding of which row to read.
PPV is always read across the
test positive row: true positives divided by the entire positive row total.
References (research sources)
- [1]
Accuracy of on-site tests to detect anemia during prenatal care.Research articleSobhy S, Rogozinska E, Khan KS (2017) · DOI: 10.1002/ijgo.12289
- [2]
Diagnostic accuracy of a non-invasive spot-check hemoglobin meter, Masimo Rad-67® pulse CO-Oximeter®, in detection of anemia in antenatal care settings in Kenya.Research articleKoech A, Mwaniki I, Mutunga J, Mukhanya M, Mwadime E, Ochieng M (2024) · DOI: 10.3389/fgwh.2024.1427261
- [3]
Comparative Diagnostic Accuracy of Clinical Pallor at Different Anatomical Sites for Detecting Anemia in Pregnant Women Using HemoCue® as the Reference Standard.Research articleJegadeesan D, Rajendran V, Rajamanickam S. (2026) · DOI: 10.7759/cureus.114293
- [4]
Diagnostic and analytic performance of four point-of-care hemoglobin testing devices used in routine HIV and maternity care in Uganda.Research articleSsuuna C, Nakawooya H, Muwanika R, Kankaka EN, Galiwango RM, Nyegenye W (2025) · DOI: 10.1186/s12913-025-13354-9