The difference between fingerstick and venous hemoglobin and hematocrit varies by sex and iron stores

Abstract

Background: Fingerstick blood samples are used to estimate donor venous hemoglobin (Hb).

Study design and methods: Fingerstick Hb or hematocrit (Hct) was determined routinely for 2425 selected donors at six blood centers, along with venous Hb. Using sex and measures of iron status including absent iron stores (AIS; ferritin < 12 ng/mL), linear regression models were developed to predict venous Hb from fingerstick.

Results: Across all subjects, fingerstick Hb was higher than venous Hb in the higher part of the clinical range, but lower in the lower part of the range. The relationship varied by sex and iron status. Across centers, a female donor had on average a venous Hb result 0.5 to 0.8 g/dL lower than a male donor with the same fingerstick Hb and iron status. Similarly, a donor with AIS had on average a venous Hb result 0.3 to 1.1 g/dL lower than an iron-replete donor with the same fingerstick value and sex. An iron-replete male donor with a fingerstick result at the cutoff (Hb 12.5 g/dL) had an acceptable expected venous Hb (12.8 to 13.8 g/dL). A female donor with AIS with a fingerstick result at the cutoff had an expected venous Hb below 12.5 g/dL (11.7 to 12.4 g/dL). Of females with AIS, 40.2% donated blood when their venous Hb was less than 12.5 g/dL.

Conclusions: Fingerstick is considered a useful estimator of venous Hb. However, in some donor groups, particularly female donors with AIS, fingerstick overestimates venous Hb at the donation cutoff. This significant limitation should be considered in setting donor fingerstick Hb or Hct requirements.

Fig. 1

Fingerstick (FS) and venous (V) Hb values observed in 2425 RISE donors, by sex and iron status. Shown are aggregated data for donors from all six centers. FS Hct values at four centers are converted to Hb by dividing by 3. Each bold line represents the median for the group. The box represents the 25th through the 75th percentile, and the lines the 2.5th through the 97.5th percentiles for each group.

Fig. 2

Linear regression model to predict venous Hb from fingerstick Hb. (A) Males at Center F. (B) Females at Center F. Each dot represents a single donor. Each line is the regression curve for one of three levels of iron stores. Line of equality is plotted for fingerstick Hb = venous Hb.